Method for identifying validated target and assay combinations for drug development

ABSTRACT

The invention comprises methods useful within a larger process for identifying compounds and/or designing further compounds with activity to produce a desired phenotype (for example, growth inhibition) in cells whose target cell component is the subject of certain studies to identify such compounds. The invention employs constructed cells comprising a regulable gene encoding a biomolecule which modulates (inhibits or activates) in vivo the function of a target component of the cell which can be an enzyme, for example, methionyl-tRNA synthetase of  Staphylococcus aureus . The process incorporates methods for identifying biomolecules that bind to a chosen target cell component in vitro, methods for identifying biomolecules that also bind to the chosen target and modulate its function intracellularly, causing a phenotypic effect. The intracellular effect of a biomolecule can be tested in cell culture, or tested after introduction of the constructed cells into a host mammal in vivo, and methods for identifying compounds that compete with the biomolecules for sites on the target in competitive binding assays. Compounds identified by the series of steps in this process are candidates for drugs with the desired activity on the cell. Targets for which such compounds can be identified are validated as being essential to a phenotype of the cell.

RELATED APPLICATIONS

This application is a continuation-in-part of U.S. patent application Ser. No. 09/291,874 filed on Apr. 14, 1999, now U.S. Pat. No. 6,436,694, which is a continuation-in-part of U.S. patent application Ser. No. 09/227,687 filed on Jan. 8, 1999, and which also claims the benefit of U.S. Provisional Application No. 60/122,949 filed on Mar. 5, 1999. This application is also a continuation-in-part of U.S. patent application Ser. No. 09/227,687 filed on Jan. 8, 1999, which claims the benefit of U.S. Provisional Application No. 60/107,751 filed on Nov. 10, 1998; U.S. Provisional Application No. 60/101,718 filed on Sep. 24, 1998; U.S. Provisional Application No. 60/100,211 filed on Sep. 14, 1998; U.S. Provisional Application No. 60/094,698 filed on Jul. 30, 1998; U.S. Provisional Application No. 60/089,828 filed on Jun. 19, 1998; U.S. Provisional Application No. 60/085,844 filed on May 18, 1998; U.S. Provisional Application No. 60/081,753 filed on Apr. 14, 1998; U.S. Provisional Application No. 60/076,638 filed on Mar. 3, 1998; and U.S. Provisional Application No. 60/070,965 filed on Jan. 9, 1998. This application also claims the benefit of U.S. Provisional Application No. 60/122,949 filed on Mar. 5, 1999. The teachings of each of these referenced applications are incorporated herein by reference in their entirety.

BACKGROUND OF THE INVENTION

In the discovery and development of new drugs, it is a common strategy to first try to identify molecules or complexes of molecules, naturally occurring within cells, that are involved in producing symptoms of a disease. These naturally occurring molecules can be thought of as “targets.” A second major part of the strategy is then to find molecules that bind to the targets. These molecules are candidates for drug development, on the theory that a molecule that binds to a target can modulate (inhibit or enhance) the function of the target, thereby causing a change in the biological status of the cell containing the target. The change caused in the cell (e.g., a change in phenotype towards wild type, or a change in growth rate) may be therapeutically beneficial to the animal or human host of the cell.

The genomics revolution, by determining the DNA sequences of great numbers of genes from many different organisms, has considerably broadened the possibilities for drug discovery by identifying large numbers of molecules that are potential targets of drug action. These technical advances in genomics however, have posed an entirely new set of challenges. Specifically, how can one prove that a chosen target molecule is essential to maintaining the disease or disorder to be treated? That is, how does one validate a target? (See “target validation” in Definitions.)

Although methods currently available to validate targets do provide some guidelines in selection of drug targets, they are usually not conducted under the conditions in which a drug actually interacts with its target, and therefore provide a limited set of information. In addition, they do not directly address, among other things: 1) if a wild type (normal) target is essential for cell growth and viability during the disease state; 2) if the wild type gene products themselves are suitable targets for drug discovery; 3) if specific sites on a target are suitable for drug interaction (for example, in a pathogenic organism, there can be one gene coding for a single protein target with two activities—one activity essential for growth and infectivity, the second activity non-essential); 4) if a compensatory mechanism in the cell, either in vitro or in vivo, can overcome or compensate for target modulation or, 5) if a disease state can be cured by modulation of function of the candidate target. These methods also do not provide a direct route for testing wild type target proteins in high throughput screening assays.

An analysis of the discovery of novel antimicrobial agents illustrates the problems researchers in all fields of drug development face today. The increasing prevalence of drug-resistant pathogens (bacteria, fungi, parasites, etc.) has led to significantly higher mortality rates from infectious diseases and currently presents a serious crisis worldwide. Despite the introduction of second and third generation antimicrobial drugs, certain pathogens, such as vancomycin resistant strains of Enterococcusfacieum, have developed resistance to all currently available drugs. New antimicrobial drugs must be discovered to treat such infections by such organisms, and new methods are urgently needed to facilitate making such discoveries.

Neither whole cell screening, chemistry nor target based drug discovery approaches as currently applied, have met the challenge of controlling infectious diseases, particularly those caused by drug resistant microorganisms. Whole cell screening assays have been limited by the fact that they are unable to identify compounds that can effectively modulate a target function inside the cell but cannot permeate the cell membrane to get to the target. Therefore entire classes of potent, intracellular target modulators, which could be subsequently modified by medicinal chemistry to increase cell membrane permeability, go undetected. Chemistry based approaches have focused on chemically modifying the molecular structure of existing antimicrobial drugs or combining existing antimicrobials with another agent to circumvent established resistance mechanisms. Technical advances in molecular biology, automated methods for high throughput screening and chemical syntheses have led to an increase in the number of target based screens utilized for antimicrobial drug discovery and in the number of compounds being analyzed. However, despite these advances, only a limited number of antimicrobial drugs acting by a novel mechanism have been identified during recent years.

How does one efficiently establish screening assays for drugs that can be used with a variety of different targets having different properties, enzymatic activities, or even unknown functions? A number of potentially novel, valuable targets are incompatible with current methods to screen for drug candidates because either the target's exact function and molecular mechanism of action are unknown, or there are technical obstacles preventing the development of effective high throughput screening methods. It can take anywhere from six months to several years to develop a screening assay, which is impractical when the goal is to rapidly screen multiple targets in a cost-effective manner.

The path in the progression from target identification through assay development, high throughput screening, medicinal chemistry, lead optimization, preclinical and clinical drug development is expensive, time consuming and full of technical challenges. Many different targets must be screened against multiple chemical compounds to identify new lead compounds for drug development. New, efficient technologies are needed that can be broadly applied to a variety of different targets to validate targets in the direct context of the desired outcome of drug therapy and to rapidly develop screening assays using these targets for drug discovery. Such developments will allow the wealth of genomics information to be leveraged for drug discovery and will lower the risk and costs while expediting the timelines of the drug discovery process.

SUMMARY OF THE INVENTION

The invention relates to methods that couple the validation of a target (see Definitions) for drug discovery with the development of an assay to identify compounds that cause a phenotypic effect on the target cell. These procedures can be applied to identifying compounds that bind to and modulate the function of target components of a cell whose function is known or unknown, and cell components that are not amenable to other screening methods.

The invention relates to procedures for identifying a compound that binds to and modulates (inhibits or enhances) the function of a component of a cell, thereby producing a phenotypic effect in the cell. Within these procedures are methods for identifying a biomolecule (See Definitions section) that 1) binds to, in vitro, a component of a cell that has been isolated from other constituents of the cell and that 2) causes, in vivo, as seen in an assay upon intracellular expression of the biomolecule, a phenotypic effect (See Definitions section) in the cell which is the usual producer and host of the target cell component. In an assay demonstrating characteristic 2) above, intracellular production of the biomolecule can be in cells grown in culture or in cells introduced into an animal. Further methods within these procedures are those methods comprising an assay for a phenotypic effect in the cell upon intracellular production of the biomolecule, either in cells in culture or in cells that have been introduced into one or more animals, and an assay to identify one or more compounds that behave as competitors of the biomolecule in an assay of binding to the target cell component.

One procedure envisioned in the invention is a process for identifying one or more compounds that produce a phenotypic effect on a cell. The process is at the same time a method for target validation (See Definitions section). The process is characterized by identifying a biomolecule which binds an isolated target cell component, constructing cells comprising the target cell component and further comprising a gene encoding the biomolecular binder which can be expressed to produce the biomolecular binder, testing the constructed cells for their ability to produce, upon expression of the gene encoding the biomolecular binder, a phenotypic effect in the cells (e.g., inhibition of growth), wherein the test of the constructed cells can be a test of the cells in culture or a test of the cells after introducing them into host animals, or both, and further, identifying, for a biomolecular binder that caused the phenotypic effect, one or more compounds that compete with the biomolecular binder for binding to the target cell component.

A test of the constructed cells after introducing them into host animals is especially well-suited to assessing whether a biomolecular binder can produce a particular phenotype by the expression (regulatable by the researcher) of a gene encoding the biomolecular binder. In this method, cells are constructed which have a gene encoding the biomolecular binder, and wherein the biomolecular binder can be produced by regulation of expression of the gene. The constructed cells are introduced into a set of animals. Expression of the gene encoding the biomolecular binder is regulated in one group of the animals (test animals) such that the biomolecular binder is produced. In another group of animals, the gene encoding the biomolecular binder is regulated such that the biomolecular binder is not produced (control animals). The cells in the two groups of animals are monitored for a phenotypic change (for example, a change in growth rate). If the phenotypic change is observed in cells in the test animals and not in the cells in the control animals, or to a lesser extent in the control animals, then the biomolecular binder has been proven to be effective in binding to its target cell component under in vivo conditions.

A further embodiment of the invention is a method for determining whether a target cell component of a particular cell type (a “first cell”) is essential to producing a phenotypic effect on the first cell, the method having the steps: isolating the target component of the first cell; identifying a biomolecular binder of the isolated target component of the first cell; constructing a second type of cells (“second cell”) comprising the target component and a regulable, exogenous gene encoding the biomolecular binder; and testing the second cell in culture for an altered phenotypic effect, upon production of the biomolecular binder in the second cell; whereby, if the second cell shows the altered phenotypic effect upon production of the biomolecular binder, then the target component of the first cell is essential to producing the phenotypic effect on the first cell. The target cell component in this embodiment and in other embodiments not limited to pathogens can be one that is found in mammalian cells, especially cells of a type found to cause or contribute to disease or the symptoms of disease (e.g., cells of tumors or cells of other types of hyperproliferative disorders).

The invention further relates to methods particularly well suited to a procedure for identifying and/or designing compounds with antimicrobial activity against a pathogen whose target cell component is the subject of studies to identify such compounds. A common mechanism of action of an antimicrobial agent is binding to a component of the cells of the pathogen treated with the antimicrobial. The procedure includes methods for identifying biomolecules that bind to a chosen target in vitro, methods for identifying biomolecules that also bind to the chosen target and modulate its function during infection of a host mammal in vivo, and methods for identifying compounds that compete with the biomolecules for sites on the target in competitive binding assays. Compounds identified by this procedure are candidates for drugs with antimicrobial activity against the pathogen.

One embodiment of the invention is a method for identifying a biomolecular inhibitor of growth of pathogen cells by using cell culture techniques, comprising contacting one or more types of biomolecules with isolated target cell component of the pathogen, applying a means of detecting bound complexes of biomolecules and target cell component, whereby, if the bound complexes are detected, one or more types of biomolecules have been identified as a biomolecular binder of the target cell component, constructing a pathogen strain having a regulable gene encoding the biomolecular binder, regulating expression of the gene encoding the biomolecular binder to express the gene; and monitoring growth of the pathogen cells in culture relative to suitable control cells, whereby, if growth of the pathogen cells is decreased compared to growth of suitable control cells, then the biomolecule is a biomolecular inhibitor of growth of the pathogen cells.

A further embodiment of the invention is a method, employing an animal test, for identifying one or more compounds that inhibit infection of a mammal by a pathogen by binding to a target cell component, comprising constructing a pathogen comprising a regulable gene encoding a biomolecule which binds to the target cell component, infecting test animals with the pathogen, regulating expression of the regulable gene to produce the biomolecule, monitoring the test animals and suitable control animals for signs of infection, wherein observing fewer or less severe signs of infection in the test animals than in suitable control animals indicates that the biomolecule is a biomolecular inhibitor of infection, and identifying one or more compounds that compete with the biomolecular inhibitor of growth for binding to the target cell component (as by employing a competitive binding assay), then the compound inhibits infection of a mammal by a pathogen by binding to a target.

The competitive binding assay to identify binding analogs of biomolecular binders, which have been proven to bind to their targets in an intracellular test of binding, can be applied to any target for which a biomolecular binder has been identified, including targets whose function is unknown or targets for which other types of assays are not easily developed and performed. Therefore, the method of the invention offers the advantage of decreasing assay development time when using a gene product of known function as a target cell component and the advantage of bypassing the major hurdle of gene function identification when using a gene product of unknown function as a target cell component.

Other embodiments of the invention are cells comprising a biomolecule and a target cell component, wherein the biomolecule is produced by expression of a regulable gene, and wherein the biomolecule modulates function of the target cell component, thereby causing a phenotypic change in the cells. Yet other embodiments are cells comprising a biomolecule and a target cell component, wherein the biomolecule is a biomolecular binder of the target cell component, and is encoded by a regulable gene. The cells can include mammalian cells or cells of a pathogen, for instance, and the phenotypic change can be a change in growth rate. The pathogen can be a species of bacteria, yeast, fungus, or parasite, for example.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is an illustration showing the steps in cloning of the Pro-3 peptide for regulated expression as a GST (glutathione-S transferase) fusion protein. See Example 1.

FIG. 2 is a photograph of an SDS-polyacrylamide gel showing expression of the Pro-3 peptide in E. coli cells. Production of the Pro-3/GST fusion and production of β-galactosidase in E. coli cells carrying pC³844 or pPROTet-LacZ were analyzed by SDS-PAGE. The concentrations of anhydrotetracycline (aTc) used for induction of gene expression are indicated above the lanes on the gel. See Example 2.

FIG. 3A is a graph showing E. coli growth inhibition by expression of Pro-3 peptide encoded on the pC³844 plasmid. The OD₆₀₀'s of the bacterial cultures in the presence (+aTc) or absence (−aTc) of anhydrotetracycline were monitored at the time points shown. See Example 2.

FIG. 3B is a graph showing results of a control experiment to compare with the results shown in FIG. 3A. The E. coli cells used in the experiment harbor the pPROTet-GST plasmid.

FIG. 4A is a graph showing functional complementation of growth inhibition by Pro-3 peptide, by expression of a heterologous ProRS gene. The growth of E. coli cells DH5αPRO/pC³844 carrying a Staphylococcus aureus ProRS expression construct was characterized in the presence (Pro3 Expression) or absence (No Pro3 Expression) of 200 ng/ml anhydrotetracycline by monitoring OD₆₀₀ at the time points shown.

FIG. 4B is a graph showing the results of a control experiment to compare with the results in FIG. 4A. The growth of E. coli cells DH5αPRO/pC³844 carrying pACYC177 was characterized in the presence (Pro3 Expression) or absence (No Pro3 Expression) of 200 ng/ml anhydrotetracycline by monitoring OD₆₀₀ at the time points shown.

FIG. 5 is a graph showing fluorescence polarization plotted with the concentration of compound added, indicating inhibition of Pro3 peptide binding to E. coli ProRS in this assay by known inhibitors: open circles, CB-16914; filled circles, CB-118831; open squares, CB-680 negative control.

FIG. 6 is a scanned image of an 18% SDS-polyacrylamide gel stained with Coomassie blue. Inducible expression of chloramphenicol acetyltransferase (CAT) in S. aureus. The whole cell lysates of S. aureus RN4220 (Host) or RN4220 harboring pWH353 (C³SaE-1) or pWH354 (C³SaE-2) with (+) or without (−) tetracycline induction were analyzed by electrophoresis on an 18% SDS-polyacrylamide gel.

FIG. 7 is a graph showing S. aureus MetRS tRNA charging activity (shown as counts per minute of trichloracetic acid precipitable [³H]-methionine) is inhibited by increasing concentrations of JT01.

FIG. 8 is a bar graph showing fluorescence of microtiter wells in the assay described in Example 9. Wells 1-6 contain Met-1F alone. Wells 7 and 8 contain S. aureus MetRS (methionyl-tRNA synthetase) alone; wells 9-18 and 19-28 contain Met-1 peptide from 640 nM to 1.25 nM; wells 29-96 contain 1% DMSO (dimethyl sulfoxide).

FIG. 9 is a diagram of the steps taken to construct plasmid pCL84tt, which was then used in the construction of S. aureus strain CYL316. Restriction sites on the plasmids are indicated as follows: E=EcoRI; H=HindIII; S=SalI. P indicates promoter.

FIG. 10 is a diagram of the steps taken to construct plasmids pC³874 and pC³875.

FIG. 11A is a graph showing the growth, as measured by OD₆₀₀, of S. aureus strain CYL316t in the presence (open diamonds) or absence (filled diamonds) of 200 ng/ml of anhydrotetracycline.

FIG. 11B is a graph showing the growth, as measured by OD₆₀₀, of S. aureus strain CYL316t/pWH353(ΔtetR) in the presence (open diamonds) or absence (filled diamonds) of 200 ng/ml of anhydrotetracycline.

FIG. 11C is a graph showing the growth, as measured by OD₆₀₀, of S. aureus strain CYL316t/pWH354(ΔtetR) in the presence (open diamonds) or absence (filled diamonds) of 200 ng/ml of anhydrotetracycline.

FIG. 11D is a graph showing the growth, as measured by OD₆₀₀, of S. aureus strain CYL316t/pWH354-2G(ΔtetR) in the presence (open diamonds) or absence (filled diamonds) of 200 ng/ml of anhydrotetracycline.

FIG. 11E is a graph showing the growth, as measured by OD₆₀₀, of S. aureus strain CYL316t/pC³859 in the presence (open diamonds) or absence (filled diamonds) of 200 ng/ml of anhydrotetracycline.

FIG. 12 is a scan of an SDS-polyacrylamide gel, stained with Coomassie blue, which was loaded with the glutathione-Sepharose-purified proteins produced under induction conditions (“I” lanes) or uninduced conditions (“U” lanes) in CYL316t/pC³878, CYL316t/pC³876, CYL316t/pC³874 and CYL316t/pC³875. Glutathione S-transferase is the prominent band.

FIG. 13A is a graph showing the growth, as measured by OD₆₀₀, of S. aureus strain CYL316tt bearing pC³875 upon induction (filled diamonds) or no induction (open diamonds) of gene expression under control of the tetracycline promoter/operator.

FIG. 13B is a graph showing the growth, as measured by OD₆₀₀, of S. aureus strain CYL316tt bearing pC³882 upon induction (filled diamonds) or no induction (open diamonds) of gene expression under control of the tetracycline promoter/operator.

FIG. 13C is a graph showing the growth, as measured by OD₆₀₀, of S. aureus strain CYL316tt bearing pC³883 upon induction (filled diamonds) or no induction (open diamonds) of gene expression under control of the tetracycline promoter/operator.

DETAILED DESCRIPTION OF THE INVENTION Definitions

Target: (also, “target component of a cell,” or “target cell component”) a constituent of a cell which contributes to and is necessary for the production or maintenance of a phenotype of the cell in which it is found. A target can be a single type of molecule or can be a complex of molecules. A target can be the product of a single gene, but can also be a complex comprising more than one gene product (for example, an enzyme comprising α and β subunits, mRNA, tRNA, ribosomal RNA or a ribonucleoprotein particle such as a snRNP). Targets can be the product of a characterized gene (gene of known function) or the product of an uncharacterized gene (gene of unknown function).

Target Validation: the process of determining whether a target is essential to the maintenance of a phenotype of the cell type in which the target normally occurs. For example, for pathogenic bacteria, researchers developing antimicrobials want to know if a compound which is potentially an antimicrobial agent not only binds to a target in vitro, but also binds to, and modulates the function of, a target in the bacteria in vivo, and especially under the conditions in which the bacteria are producing an infection—those conditions under which the antimicrobial agent must work to inhibit bacterial growth in an infected animal or human. If such compounds can be found that bind to a target in vitro and alter the target's function in cells resulting in an altered phenotype, as found by testing cells in culture and/or as found by testing cells in an animal, then the target is validated.

Phenotypic Effect: a change in an observable characteristic of a cell which can include, e.g., growth rate, level or activity of an enzyme produced by the cell, sensitivity to various agents, antigenic characteristics, and level of various metabolites of the cell. A phenotypic effect can be a change away from wild type (normal) phenotype, or can be a change towards wild type phenotype, for example. A phenotypic effect can be the causing or curing of a disease state, especially where mammalian cells are referred to herein. For cells of a pathogen or tumor cells, especially, a phenotypic effect can be the slowing of growth rate or cessation of growth.

Biomolecule: a molecule which can be produced as a gene product in cells that have been appropriately constructed to comprise one or more genes encoding the biomolecule. Preferably, production of the biomolecule can be turned on, when desired, by an inducible promoter. A biomolecule can be a peptide, polypeptide, or an RNA or RNA oligonucleotide, a DNA or DNA oligonucleotide, but is preferably a peptide. The same biomolecules can also be made synthetically. For peptides, see Merrifield, J., J. Am. Chem. Soc. 85: 2140-2154 (1963). For instance, an Applied Biosystems 431A Peptide Synthesizer (Perkin Elmer) can be used for peptide synthesis. Biomolecules produced as gene products intracellularly are tested for their interaction with a target in the intracellular steps described herein (tests performed with cells in culture and tests performed with cells that have been introduced into animals). The same biomolecules produced synthetically are tested for their binding to an isolated target in an initial in vitro method described herein. Synthetically produced biomolecules can also be used for a final step of the method for finding compounds that are competitive binders of the target.

Biomolecular Binder (of a target): a biomolecule which has been tested for its ability to bind to an isolated target cell component in vitro and has been found to bind to the target.

Biomolecular Inhibitor of Growth: a biomolecule which has been tested for its ability to inhibit the growth of cells constructed to produce the biomolecule in an “in culture” test of the effect of the biomolecule on growth of the cells, and has been found, in fact, to inhibit the growth of the cells in this test in culture.

Biomolecular Inhibitor of Infection: a biomolecule which has been tested for its ability to ameliorate the effects of infection, and has been found to do so. In the test, pathogen cells constructed to regulably express the biomolecule are introduced into one or more animals, the gene encoding the biomolecule is regulated so as to allow production of the biomolecule in the cells, and the effects of production of the biomolecule are observed in the infected animals compared to one or more suitable control animals.

Isolated: term used herein to indicate that the material in question exists in a physical milieu distinct from that in which it occurs in nature. For example, an isolated target cell component of the invention may be substantially isolated with respect to the complex cellular milieu in which it naturally occurs. The absolute level of purity is not critical, and those skilled in the art can readily determine appropriate levels of purity according to the use to which the material is to be put. In many circumstances the isolated material will form part of a composition (for example, a more or less crude extract containing other substances), buffer system or reagent mix. In other circumstances, the material may be purified to essential homogeneity, for example as determined by PAGE or column chromatography (for example, HPLC).

Pathogen or Pathogenic Organism: an organism which is capable of causing disease, detectable by signs of infection or symptoms characteristic of disease. Pathogens can include procaryotes (which include, for example, medically significant Gram-positive bacteria such as Streptococcus pneumoniae, Enterococcus faecalis and Staphylococcus aureus, Gram-negative bacteria such as Escherichia coli, Pseudomonas aeroginosa and Klebsiella pneumoniae, and “acid-fast” bacteria such as Mycobacteria, especially M. tuberculosis), eucaryotes such as yeast and fungi (for example, Candida albicans and Aspergillus fumigatus) and parasites. It should be recognized that pathogens can include such organisms as soil-dwelling organisms and “normal flora” of the skin, gut and orifices, if such organisms colonize and cause symptoms of infection in a human or other mammal, by abnormal proliferation or by growth at a site from which the organism cannot usually be cultured.

The present invention relates to methods that couple the validation of a target cell component for drug discovery with the development of a validated assay to identify compounds that cause a phenotypic effect on the target cell (cell harboring the target cell component). When the target cells are cells of a pathogenic organism, compounds identified by this procedure are candidates for drugs with antimicrobial activity against the pathogen.

The method utilized for target validation provides a test of how a biomolecule produced intracellularly binds to a specific site on a target cell component and alters the target's function in a cell during an established infection or disease. The technology to validate the target identifies a biomolecule specific to the target that can be used in a screening assay to identify drug leads, thereby coupling target validation with drug lead identification. The method also validates specific sites on a target molecule for drug discovery, which is especially important for proteins involved in multiple functions.

Described herein are methods that result in the identification of compounds that cause a phenotypic effect on a cell. The general steps described herein to find a compound for drug development can be thought of as these: (1) identifying a biomolecule that can bind to an isolated target cell component in vitro, (2) confirming that the biomolecule, when produced in cells with the target cell component, can cause a desired phenotypic effect and (3) identifying, by an in vitro screening method, for example, compounds that compete with the biomolecule for binding to the target cell component. Advantages of the these steps are that it is not necessary to identify the function of the target cell component and it is not necessary to develop an assay tailored to the function (e.g., enzyme activity) of the target cell component.

Central to these methods is general step (2) above, intracellular validation of a biomolecule comprising one or more steps that determine whether a biomolecule can cause a phenotypic effect on a cell, when the biomolecule is produced by the expression (which can be regulable) of a gene in the cell. As used in general step (2), a biomolecule is a gene product (e.g., polypeptide, RNA, peptide or RNA oligonucleotide) of an exogenous gene—a gene which has been introduced in the course of construction of the cell. See also Definitions section.

Biomolecules that bind to and alter the function of a candidate target are identified by various in vitro methods. Upon production of the biomolecule within a cell either in vitro or within an animal model system, the biomolecule binds to a specific site on the target, alters its intracellular function, and hence produces a phenotypic change (e.g. cessation of growth, cell death). When the biomolecule is produced in engineered pathogen cells in an animal model of infection, cessation of growth or death of the engineered pathogen cells leads to the clearing of infection and animal survival, demonstrating the importance of the target in infection and thereby validating the target.

A method for (1) identifying a biomolecule that produces a phenotypic effect on a cell (wherein the cell can be, for instance, a pathogen cell or a mammalian cell) and (2) simultaneous intracellular target validation, can comprise steps of introducing into an animal a cell comprising an exogenous regulable gene encoding the biomolecule, regulating expression of the gene to produce the biomolecule in the cell, and monitoring said cell in the animal for a phenotypic effect, compared to a suitable control cell. If the cell of this test manifests a phenotypic effect, this indicates that the biomolecule produced in the cell causes a phenotypic effect on the cell. If this phenotypic effect is the inhibition of growth of the cells, then the biomolecule can be termed a “biomolecular inhibitor” or a “biomolecular inhibitor of growth.” It may be desirable to perform another test of intracellular function, using cell culturing techniques, wherein the cell comprising an exogenous regulable gene encoding the biomolecule of interest, and comprising the target cell component, is treated so as to turn on expression of the gene encoding the biomolecule, and one or more phenotypic characteristics of the cells in culture are monitored relative to suitable control cells, where the control cells do not produce the biomolecule. It may be preferable, where both “in culture” and “in animal” intracellular tests are performed, to do an “in culture” test first.

The purpose of intracellular validation for the combination of a potential target for drug action and molecule for drug development is two-fold. First, it demonstrates that the biomolecule under study produces a phenotypic effect on a living cell. In contrast with conditions in an in vitro binding test, the biomolecule in an intracellular test is exposed to a multitude of potential binding partners in the living cell, and interaction with one or more of these binding partners in the cell may be unproductive or result in undesirable effects. These effects are not detectable in an in vitro binding test. Second, where a biomolecule has been shown previously by in vitro tests to bind to a target cell component (that is, the biomolecule can be called a “biomolecular binder” of the target cell component; See Definitions section), intracellular validation provides proof that the target cell component is essential to the maintenance of the original phenotype of the cell. Therefore, the target is validated for drug discovery and the biomolecule can then be utilized in a competitive binding assay to identify compounds that will have an effect on target molecule function.

Efficient binding between a biomolecule and a target cell component may be demonstrated in vitro; even binding that inhibits activity of a target enzyme may be demonstrated in vitro. However, in the living cells, there could exist a redundant system that nullifies the effect of the biomolecule binding to the target cell component. For example, production of an enzyme having similar activity to that of the target cell component may be induced in the cells. By a mechanism such as this, the cell could escape any effect the biomolecule might otherwise cause by binding to the target cell component.

Using an intracellular test to validate biomolecule/target cell component interaction is superior to using only an in vitro test using isolated molecules, because the intracellular test ensures that the target cell component is in its natural conformation and that the biomolecule “sees” the target cell component in that conformation, as that conformation occurs in a disease state. That in an intracellular test a biomolecule finds a site which ultimately causes a phenotypic effect on the cell indicates that the biomolecule is binding to a functionally relevant site on the target cell component (e.g., an active site of an enzyme). Thus, molecules that are found to be structural analogs of the biomolecule and to compete with the biomolecule for a binding site on the target will also interact with the functionally relevant site of the target cell component, as functional analogs. A functional analog of the biomolecule can be found through competitive binding assays of the biomolecule against compounds (as in a library of compounds) that are potential binders of the target cell component. Structural analogs can also be found by rational drug design once a biomolecular binder is identified, by designing drugs that mimic the structure of the biomolecular binder. These structural analogs can be tested for their binding properties by techniques described herein.

A further advantage of the intracellular test in which the biomolecule is produced from one or more genes in the cell, is that the biomolecule does not have to pass through a cell membrane or rely on inefficient uptake mechanisms of the cell. Intracellular production of the biomolecule ensures that a biomolecule that interacts with a functional site on a target cell component to produce an effect will be detected, even if uptake of the biomolecule into the cell is limited. By the intracellular test, more biomolecules testing as being able to cause the desired phenotypic effect can be detected as candidates for further testing to find functional analogs for drug development. In a test employing extracellular addition of biomolecules, biomolecules that bind the target cell component but are taken up by the cell only to a limited extent could be missed as candidates for further testing to find functional analogs for drug development. Limited uptake of a biomolecule which has been found to bind to a target in vitro is not necessarily a barrier to further steps towards drug development, as a structural portion of the ultimate compound to be administered as a drug can be selected for its stability, membrane solubility, efficient uptake, etc., and can be chemically combined with a compound whose structure mimics the active binding portion of the biomolecule. Intracellular production of the biomolecule, in an intracellular test of the effect of a biomolecule, as opposed to uptake from outside the cell, can also minimize degradation of the biomolecules from extracellular and intracellular degradative enzymes (e.g., proteases).

In further steps following one or more intracellular tests of the biomolecule/target cell component combination, one or more compounds that can also produce the phenotypic effect caused by the biomolecule can be identified in an in vitro competitive binding assay (which may be adapted for high-throughput screening) as compounds that compete with the biomolecule for a binding site on the target cell component. Target cell components can be isolated from the type of cell in which the phenotypic effect is desired (for instance, cells of pathogenic bacteria, yeast or fungi; mammalian cells, such as tumor cells), or from cells engineered to produce the cell component or a derivative of the cell component that would provide (at least some) structurally identical binding sites (e.g., a fusion protein). Compounds that produce the phenotypic effect observed with the biomolecule can be found in the competitive binding assay upon screening of libraries of compounds (for example, small molecule compounds or natural products or libraries that can be selected for having as their members compounds that have greater intracellular stability than biomolecules such as peptides or RNA oligonucleotides).

The invention includes methods for identifying compounds that inhibit the growth of cells having a target cell component. The target cell component can first be identified as essential to the growth of the cells in culture and/or under conditions in which it is desired that the growth of the cells be inhibited. These methods can be applied, for example, to various types of cells that undergo abnormal or undesirable proliferation, including cells of neoplasms (tumors or growths, either benign or malignant) which, as known in the art, can originate from a variety of different cell types. Such cells can be referred to, for example, as being from adenomas, carcinomas, lymphomas or leukemias. The method can also be applied to cells that proliferate abnormally in certain other diseases, such as arthritis, psoriasis or autoimmune diseases.

Described herein are similar methods for identifying inhibitors of target molecules or target cell components of pathogenic organisms. These methods can include a target validation procedure using an animal model for confirming that a cell component of a pathogenic organism is essential, after infection with the organism has been established in a host, and that the inhibitor is effective against the organism after the organism has established the infection. A goal of the procedure is to identify compounds and/or gain the knowledge required to design compounds that can be used as antimicrobial agents to treat a human or other mammal having an infection of the organism.

The invention provides methods for in vitro and in vivo validation of target and assay combinations. Following selection of biomolecular binders to the isolated target cell component of interest, the invention can incorporate steps for (1) regulable (e.g., inducible) intracellular expression of a gene encoding the biomolecular binder and (2) monitoring cell viability in culture (e.g., cell growth in liquid media or agar plates) or in vivo (e.g., growth of introduced cells or pathogen virulence in an animal infection model) or both. If intracellular expression of the biomolecular binder inhibits the function of a target essential for growth (presumably by binding to the target at a biologically relevant site), cells monitored in step (2) will exhibit a slow growth or no growth phenotype. Targets found to be essential for growth by these methods are validated starting points for drug discovery, and can be incorporated into assays to identify more stable compounds that bind to the same site on the target as the biomolecule.

Where the cells are pathogen cells and the desired phenotypic change to be monitored is inhibition of growth, the invention provides a procedure to examine the activity of target (pathogen) cell components in an animal infection model. Controlled expression in cells of biomolecular binders to the target of interest mimics the environment for traditional antimicrobial therapy and validates targets as essential and appropriate for drug discovery. The technology facilitates choosing the best antimicrobial targets for drug discovery by facilitating direct observation of the effect (phenotype) produced by target inhibition at a specific target subsite. The process is broadly applicable to a variety of targets. The process also validates target and biomolecular binder combinations as a direct path to high throughput screening for binding analogs of the biomolecular binder, and is equally facile with targets that are gene products of genes of unknown function or genes of known function. Validated target and biomolecular binder assay combinations can be used directly in in vitro or in vivo competitive binding assays for screening chemical compound files. Compounds that compete with the biomolecular binders are identified as potential medicinal chemistry leads.

In the course of this method, it may be decided to study as a target cell component a gene product of a particular cell type (e.g., a type of pathogenic bacteria, such as Staphylococcus aureus), wherein the target cell component (for example, an aminoacyl-tRNA synthetase such as methionyl-tRNA synthetase) is already known as being encoded by a characterized gene, as a potential target for a modulator to be identified. In this case, the target cell component can be isolated directly from the cell type of interest, assuming suitable culture methods are available to grow a sufficient number of cells, using methods appropriate to the type of cell component to be isolated (e.g., protein purification methods such as differential precipitation, ion exchange chromatography, gel chromatography, affinity chromatography, HPLC). Alternatively, the target cell component can be produced recombinantly, which requires that the gene encoding the target cell component be isolated from the cell type of interest. This can be done by any number of methods, for example known methods such as PCR, using template DNA isolated from the pathogen or a DNA library produced from the pathogen DNA, and using primers based on known sequences or combinations of known and unknown sequences within or external to the chosen gene. See, for example, methods described in “The Polymerase Chain Reaction,” Chapter 15 of Current Protocols in Molecular Biology, (Ausubel, F. M. et al, eds), John Wiley & Sons, New York, 1998. Other methods include cloning a gene from a DNA library (e.g., a cDNA library from a eucaryotic pathogen) into a vector (e.g., plasmid, phage, phagemid, virus, etc.) and applying a means of selection or screening to clones resulting from a transformation of vectors (including a population of vectors now having inserted genes) into appropriate host cells. The screening method can take advantage of properties given to the host cells by the expression of the inserted chosen gene (e.g., detection of the gene product by antibodies directed against it, detection of an enzymatic activity of the gene product), or can detect the presence of the gene itself (for instance, by methods employing nucleic acid hybridization). For methods of cloning genes in E. coli, which also may be applicable to cloning in other bacterial species, see, for example, “Escherichia coli, Plasmids and Bacteriophages,” Chapter 1 of Current Protocols in Molecular Biology, (Ausubel, F. M. et al., eds), John Wiley & Sons, New York, 1998. For methods applicable to cloning genes of eukaryotic origin, see Chapter 5 (“Construction of Recombinant DNA Libraries”), Chapter 9 (“Introduction of DNA Into Mammalian Cells”) and Chapter 6 (“Screening of Recombinant DNA Libraries”) of Current Protocols in Molecular Biology, (Ausubel, F. M. et al., eds), John Wiley & Sons, New York, 1998.

Target proteins can be expressed with E. coli or other prokaryotic gene expression systems, or in eukaryotic gene expression systems. Since many eukaryotic proteins carry unique modifications that are required for their activities, e.g. glycosylation and methylation, protein expression can in some cases be better carried out in eukaryotic systems, such as yeast, insect, or mammalian cells that can perform these modifications. Examples of these expression systems have been reviewed in the following literature: Methods in Enzymology, Volume 185, eds D. V. Goeddel, Academic Press, San Diego, 1990; Geisse et al, Protein Expression and Purification 8:271-282, 1996; Simonsen and McGrogan, Biologicals 22: 85-94; Jones and Morikawa, Current Opinions in Biotechnologies 7: 512-516, 1996; Possee, Current Opinions in Biotechnologies 8:569-572.

Where a gene encoding a chosen target cell component has not been isolated previously, but is thought to exist because homologs of the gene product are known in other species, the gene can be identified and cloned by a method such as that used in Shiba et al., U.S. Pat. No. 5,759,833, Shiba et al., U.S. Pat. No. 5,629,188, Martinis et al., U.S. Pat. No. 5,656,470 and Sassanfar et al., U.S. Pat. No. 5,756,327. The teachings of these four patents are incorporated herein by reference in their entirety.

It is an advantage of the target validation method that it can be used with target cell components which have not been previously isolated or characterized and whose functions are unknown. In this case, a segment of DNA containing an open reading frame (ORF; a cDNA can also be used, as appropriate to a eukaryotic cell) which has been isolated from a cell of a type that is to be an object of drug action (e.g., tumor cell, pathogen cell) can be cloned into a vector, and the target gene product of the ORF can be produced in host cells harboring the vector. The gene product can be purified and further studied in a manner similar to that of a gene product that has been previously isolated and characterized.

In some cases, the open reading frame (in some cases, cDNA) can be isolated from a source of DNA of the cells of interest (genomic DNA or a library, as appropriate), and inserted into a fusion protein or fusion polypeptide construct. This construct can be a vector comprising a nucleic acid sequence which provides a control region (e.g., promoter, ribosome binding site) and a region which encodes a peptide or polypeptide portion of the fusion polypeptide wherein the polypeptide encoded by the fusion vector endows the fusion polypeptide with one or more properties that allow for the purification of the fusion polypeptide. For example, the vector can be one from the pGEX series of plasmids (Pharmacia) designed to produce fusions with glutathione S-transferase.

The isolated DNA having an open reading frame, whether encoding a known or an as yet unidentified gene product, when inserted into an expression construct, can be expressed to produce the target cell component in host cells. Host cells can be, for example, Gram-negative or Gram-positive bacterial cells such as Escherichia coli or Bacillus subtilis, respectively, or yeast cells such as Saccharomyces cerevisiae, Schizosaccharomyces pombe or Pichia pastoris. It is preferable that the target cell component to be used in target validation studies be produced in a host that is genetically related to the pathogen from which the gene encoding it was isolated. For example, for a Gram-negative bacterial pathogen, an E. coli host is preferred over a Pichia pastoris host. The target cell component so produced can then be isolated from the host cells. Many protein purification methods are known that separate proteins on the basis of, for instance, size, charge, or affinity for a binding partner (e.g., for an enzyme, a binding partner can be a substrate or substrate analog), and these methods can be combined in a sequence of steps by persons of skill in the art to produce an effective purification scheme. For methods to manipulate RNA, see, for example, Chapter 4 in Current Protocols in Molecular Biology (Ausubel, F. M. et al., eds), John Wiley & Sons, New York, 1998.

An isolated cell component or a fusion protein comprising the cell component can be used in a test to identify one or more biomolecular binders of the isolated product (general step (1)). A biomolecular binder of a target cell component (See Definitions section) can be identified by in vitro assays that test for the formation of complexes of target and biomolecular binder noncovalently bound to each other. For example, the isolated target can be contacted with one or more types of biomolecules under conditions conducive to binding, the unbound biomolecules can be removed from the targets, and a means of detecting bound complexes of biomolecules and targets can be applied. The detection of the bound complexes can be facilitated by having either the potential biomolecular binders or the target labeled or tagged with an adduct that allows detection or separation (e.g., radioactive isotope or fluorescent label; streptavidin, avidin or biotin affinity label). Alternatively, both the potential biomolecular binders and the target can be differentially labeled. For examples of such methods see, e.g., WO 98/19162.

The biomolecules to be tested for binding to a target can be from a library of candidate biomolecular binders, (e.g., a peptide or oligonucleotide library). For example, a peptide library can be displayed on the coat protein of a phage (see, for examples of the use of genetic packages such as phage display libraries, Koivunen, E. et al., J. Biol. Chem. 268:20205-20210 (1993)). The biomolecules can be detected by means of a chemical tag or label attached to or integrated into the biomolecules before they are screened for binding properties. For example, the label can be a radioisotope, a biotin tag, or a fluorescent label. Those molecules that are found to bind to the target molecule can be called biomolecular binders.

An isolated target cell component, an antigenically similar portion thereof, or a suitable fusion protein comprising all of or a portion of or the entire target can be used in a method to select and identify biomolecules which bind specifically to the target. Where the target cell component comprises a protein, fusion proteins comprising all of, or a portion of, the target linked to a second moiety not occurring in the target as found in nature, can be prepared for use in another embodiment of the method. Suitable fusion proteins for this purpose include those in which the second moiety comprises an affinity ligand (e.g., an enzyme, antigen, epitope). The fusion proteins can be produced by the insertion of a gene encoding a target or a suitable portion of such gene into a suitable expression vector, which encodes an affinity ligand (e.g., pGEX-4T-2 and pET-15b, encoding glutathione S-transferase and His-Tag affinity ligands, respectively). The expression vector can be introduced into a suitable host cell for expression. Host cells are lysed and the lysate, containing fusion protein, can be bound to a suitable affinity matrix by contacting the lysate with an affinity matrix under conditions sufficient for binding of the affinity ligand portion of the fusion protein to the affinity matrix.

In one embodiment, the fusion protein can be immobilized on a suitable affinity matrix under conditions sufficient to bind the affinity ligand portion of the fusion protein to the matrix, and is contacted with one or more candidate biomolecules (e.g., a mixture of peptides) to be tested as biomolecular binders, under conditions suitable for binding of the biomolecules to the target portion of the bound fusion protein. Next, the affinity matrix with bound fusion protein can be washed with a suitable wash buffer to remove unbound biomolecules and non-specifically bound biomolecules. Biomolecules which remain bound can be released by contacting the affinity matrix with fusion protein bound thereto with a suitable elution buffer. Wash buffer can be formulated to permit binding of the fusion protein to the affinity matrix, without significantly disrupting binding of specifically bound biomolecules. In this aspect, elution buffer can be formulated to permit retention of the fusion protein by the affinity matrix, but can be formulated to interfere with binding of the test biomolecule(s) to the target portion of the fusion protein. For example, a change in the ionic strength or pH of the elution buffer can lead to release of biomolecules, or the elution buffer can comprise a release component or components designed to disrupt binding of biomolecules to the target portion of the fusion protein.

Immobilization can be performed prior to, simultaneous with, or after contacting the fusion protein with biomolecule, as appropriate. Various permutations of the method are possible, depending upon factors such as the biomolecules tested, the affinity matrix-ligand pair selected, and elution buffer formulation. For example, after the wash step, fusion protein with biomolecules bound thereto can be eluted from the affinity matrix with a suitable elution buffer (a matrix elution buffer, such as glutathione for a GST fusion). Where the fusion protein comprises a cleavable linker, such as a thrombin cleavage site, cleavage from the affinity ligand can release a portion of the fusion with the biomolecules bound thereto. Bound biomolecule can then be released from the fusion protein or its cleavage product by an appropriate method, such as extraction.

One or more candidate biomolecular binders can be tested simultaneously. Where a mixture of biomolecules is tested, the biomolecules selected by the foregoing processes can be separated (as appropriate) and identified by suitable methods (e.g., PCR, sequencing, chromatography). Large libraries of biomolecules (e.g., peptides, RNA oligonucleotides) produced by combinatorial chemical synthesis or other methods can be tested (see e.g., Ohlmeyer, M. H. J. et al., Proc. Natl. Acad. Sci. USA 90:10922-10926 (1993) and DeWitt, S. H. et al., Proc. Natl. Acad. Sci. USA 90:6909-6913 (1993), relating to tagged compounds; see also Rutter, W. J. et al. U.S. Pat. No. 5,010,175; Huebner, V. D. et al., U.S. Pat. No. 5,182,366; and Geysen, H. M., U.S. Pat. No. 4,833,092). Random sequence RNA libraries (see Ellington, A. D. et al., Nature 346:818-822 (1990); Bock, L. C. et al., Nature 355:584-566 (1992); and Szostak, J. W., Trends in Biochem. Sci. 17:89-93 (March, 1992)) can also be screened according to the present method to select RNA molecules which bind to a target. Where biomolecules selected from a combinatorial library by the present method carry unique tags, identification of individual biomolecules by chromatographic methods is possible. Where biomolecules do not carry tags, chromatographic separation, followed by mass spectrometry to ascertain structure, can be used to identify individual biomolecules selected by the method, for example.

Other methods to identify biomolecular binders of a target cell component can be used. For example, the two-hybrid system or interaction trap is an in vivo system that can be used to identify polypeptides, peptides or proteins (candidate biomolecular binders) that bind to a target protein. In this system, both candidate biomolecular binders and target cell component proteins are produced as fusion proteins. The two-hybrid system and variations on it have been described (U.S. Pat. Nos. 5,283,173 and 5,468,614; Golemis, E. A. et al., pages 20.1.1-20.1.35 In Current Protocols in Molecular Biology, F. M. Ausubel et al., eds., John Wiley and Sons, containing supplements up through Supplement 40, 1997; two-hybrid systems available from Clontech, Palo Alto, Calif.).

Once one or more biomolecular binders of a cell component have been identified, further steps can be combined with those taken to identify the biomolecular binder, to identify those biomolecular binders that produce a phenotypic effect on a cell (where “a cell” can mean cells of a cell strain or cell line). Thus, a method for identifying a biomolecule that produces a phenotypic effect on a first cell can comprise the steps of identifying a biomolecular binder of an isolated target cell component of the first cell; constructing a second cell comprising the target cell component and a regulable exogenous gene encoding the biomolecular binder; and testing the second cell for the phenotypic effect, upon production of the biomolecular binder in the second cell, where the second cell can be maintained in culture or introduced into an experimental animal. If the second cell shows the phenotypic effect upon intracellular production of the biomolecular binder, then a biomolecule that produces a phenotypic effect on the first cell has been identified. Testing the second cell is general step (2) of the invention, as the three general steps were outlined above.

Host cells (also, “second cells” in the terminology used above) of the cell type (e.g., species of pathogenic bacteria) the target was isolated from (or the gene encoding the target was originally isolated from, if the target is produced by recombinant methods), can be engineered to harbor a gene that can regulably express the biomolecular binder (e.g., under an inducible or repressible promoter). The ability to regulate the expression of the biomolecular binder is desirable because constitutive expression of the biomolecular binder could be lethal to the cell. Therefore, inducible or regulated expression gives the researcher the ability to control if and when the biomolecular binder is expressed. The gene expressing the biomolecular binder can be present in one or more copies, either on an extrachromosomal structure, such as on a single or multicopy plasmid, or integrated into the host cell genome. Plasmids that provide an inducible gene expression system in pathogenic organisms can be used. For example, plasmids allowing tetracycline-inducible expression of a gene in Staphylococcus aureus have been developed. See Examples.

For intracellular expression of a biomolecule to be tested for its phenotypic effect in a eukaryotic cell (e.g., mammalian cell), the genes for expression can be carried on plasmid-based or virus-based vectors, or on a linear piece of DNA or RNA. For examples of expression vectors, see Hosfield and Lu, Biotechniques 25:306-309, 1998; Stephens and Cockett, Nucleic Acid Research 17:7110, 1989; Wohlgemuth et al, Gene Therapy 3:503-512, 1996; Ramirez-Solis et al, Gene 87:291-294, 1990; Dirks et al, Gene 149:387-388, 1994; Chengalvala et al. Current Opinion in Biotechnologies 2:718-722, 1991; Methods in Enzymology, Volume 185, (D. V. Goeddel, ed.) Academic Press, San Diego, 1990. The genetic material can be introduced into cells using a variety of techniques, including whole cell or protoplast transformation, electroporation, calcium phosphate-DNA precipitation or DEAE-Dextran transfection, liposome mediated DNA or RNA transfer, or transduction with recombinant viral or retroviral vectors. Expression of the gene can be constitutive (e.g., ADH1 promoter for expression in S. cerevisiae (Bennetzen, J. L. and Hall, B. D., J. Biol. Chem. 257:3026-3031 (1982)), or CMV immediate early promoter and RSV LTR for mammalian expression) or inducible, as the inducible GAL1 promoter in yeast (Davis, L. I. and Fink, G. R., Cell 61:965-978 (1990)). A variety of inducible systems can be utilized, for example, E. coli Lac repressor/operator system and Tn10 Tet repressor/operator systems have been engineered to govern regulated expression in organisms from bacterial to mammalian cells. Regulated gene expression can also be achieved by activation. For example, gene expression governed by HIV LTR can be activated by HIV or SIV Tat proteins in human cells; GAL4 promoter can be activated by galactose in a nonglucose-containing medium. The location of the biomolecule binder genes can be extrachromosomal or chromosomally integrated. The chromosome integration can be mediated through homologous or nonhomologous recombinations.

For proper localization in the cells, it may be desirable to tag the biomolecule binders with certain peptide signal sequences (for example, nuclear localization signal (NLS) sequences, mitochondria localization sequences). Secretion sequences have been well documented in the art.

For presentation of the biomolecular binders in the intracellular system, they can be fused N-terminally, C-terminally, or internally in a carrier protein (if the biomolecular binder is a peptide), and can be fused (5′, 3′ or internally) in a carrier RNA or DNA molecule (if the biomolecular binder is a nucleic acid). The biomolecular binder can be presented with a protein or nucleic acid structural scaffold. Certain linkages (e.g., a 4-glycine linker for a peptide or a stretch of A's for an RNA can be inserted between the biomolecular binder and the carrier proteins or nucleic acids.

In such engineered cells, the effect of this biomolecular binder on the phenotype of the cells can be tested, as a manifestation of the binding (implying binding to a functionally relevant site, thus, an activator, or more likely, an inhibitory) effect of the biomolecular binder on the target used in an in vitro binding assay as described above. An intracellular test can not only determine which biomolecular binders have a phenotypic effect on the cells, but at the same time can assess whether the target in the cells is essential for maintaining the normal phenotype of the cells. For example, a culture of the engineered cells expressing a biomolecular binder can be divided into two aliquots. The first aliquot (“test” cells) can be treated in a suitable manner to regulate (e.g., induce or release repression of, as appropriate) the gene encoding the biomolecular binder, such that the biomolecular binder is produced in the cells. The second aliquot (“control” cells) can be left untreated so that the biomolecular binder is not produced in the cells. In a variation of this method of testing the effect of a biomolecular binder on the phenotype of the cells, a different strain of cells, not having a gene that can express the biomolecular binder, can be used as control cells. The phenotype of the cells in each culture (“test” and “control” cells grown under the same conditions, other than the expression of the biomolecular binder), can then be monitored by a suitable means (e.g., enzymatic activity, monitoring a product of a biosynthetic pathway, antibody to test for presence of cell surface antigen, etc.). Where the change in phenotype is a change in growth rate, the growth of the cells in each culture (“test” and “control” cells grown under the same conditions, other than the expression of the biomolecular binder), can be monitored by a suitable means (e.g., turbidity of liquid cultures, cell count, etc). If the extent of growth or rate of growth of the test cells is less than the extent of growth or rate of growth of the control cells, then the biomolecular binder can be concluded to be an inhibitor of the growth of the cells, or a biomolecular inhibitor.

If the phenotype of the test cells is altered relative to that of the control cells, then the biomolecular binder can be concluded to be one that causes a phenotypic effect. In an optional additional test, isolated target cell component having a known function (e.g., an enzyme activity) can be tested for modulation of this known function in the presence of biomolecular binder under conditions conducive to binding of the biomolecular binder to the target cell component. Positive results in these tests should encourage the investigator to continue in the drug discovery process with efforts to find a more stable compound (than a peptide, polypeptide or RNA biomolecule) that mimics the binding properties of the biomolecular binder on the tested target cell component.

A further test can, again, employ an engineered strain of cells that comprise both the target cell component and one or more genes encoding a biomolecule tested to be a biomolecular binder of the target cell component. The cells of the cell strain can be tested in animals to see if regulable expression of the biomolecular binder in the engineered cells produces an observable or testable change in phenotype of the cells. Both the “in culture” test for the effect of intracellular expression of the biomolecular binder and the “in animal” test (described below) for the effect of intracellular expression of the biomolecular binder can be applied not only towards drug discovery in the categories of antimicrobials and anticancer agents, but also towards the discovery of therapeutic agents to treat inflammatory diseases, cardiovascular diseases, diseases associated with metabolic pathways, and diseases associated with the central nervous system, for example.

Where the engineered strain of cells is a strain of pathogen cells or tumor cells, the object of the test is to see whether production of the biomolecular binder in the engineered strain inhibits growth of these cells after their introduction into an animal by the engineered pathogen. Such a test can not only determine which biomolecular binders are inhibitors of growth of the cells, but at the same time can assess whether the target in the cells is essential for maintaining growth of the cells (infection, for a pathogenic organism) in a host mammal. Suitable animals for such an experiment are, for example, mammals such as mice, rats, rabbits, guinea pigs, dogs, pigs, and the like. Small mammals are preferred for reasons of convenience. The engineered cells are introduced into one or more animals (“test” animals) and into one or more animals in a separate group (“control” animals) by a route appropriate to cause symptoms of systemic or local growth of the engineered cells. The route of introduction may be, for example, by oral feeding, by inhalation, by subdermal, intramuscular, intravenous, or intraperitoneal injection as appropriate to the desired result.

After the cell strain has been introduced into the test and control animals, expression of the gene encoding the biomolecular binder is regulated to allow production of the biomolecular binder in the engineered pathogen cells. This can be achieved, for instance, by administering to the test animals a treatment appropriate to the regulation system built into the cells, to cause the gene encoding the biomolecular binder to be expressed. The same treatment is not administered to the control animals, but the conditions under which they are maintained are otherwise identical to those of the test animals. The treatment to express the gene encoding the biomolecular binder can be the administration of an inducer substance (where expression of the biomolecular binder or gene is under the control of an inducible promoter) or the functional removal of a repressor substance (where expression of the biomolecular binder gene is under the control of a repressible promoter).

After such treatment, the test and control animals can be monitored for a phenotypic effect in the introduced cells. Where the introduced cells are constructed pathogen cells, the animals can be monitored for signs of infection (as the simplest endpoint, death of the animal, but also e.g., lethargy, lack of grooming behavior, hunched posture, not eating, diarrhea or other discharges; bacterial titer in samples of blood or other cultured fluids or tissues). In the case of testing engineered tumor cells, the test and control animals can be monitored for the development of tumors or for other indicators of the proliferation of the introduced engineered cells. If the test animals are observed to exhibit less growth of the introduced cells than the control animals, then the biomolecule can be also called a biomolecular inhibitor of growth, or biomolecular inhibitor of infection, as appropriate, as it can be concluded that the expression in vivo of the biomolecular inhibitor is the cause of the relative reduction in growth of the introduced cells in the test animals.

Further steps of the procedure involve in vitro assays to identify one or more compounds that have binding and activating or inhibitory properties that are similar to those of the biomolecules which have been found to have a phenotypic effect, such as inhibition of growth. That is, compounds that compete for binding to a target cell component with the biomolecule would then be structural analogs of the biomolecules. Assays to identify such compounds can take advantage of known methods to identify competing molecules in a binding assay. These steps comprise general step (3) of the method.

In one method to identify such compounds, a biomolecular inhibitor (or activator) can be contacted with the isolated target cell component to allow binding, one or more compounds can be added to the milieu comprising the biomolecular inhibitor and the cell component under conditions that allow interaction and binding between the cell component and the biomolecular inhibitor, and any biomolecular inhibitor that is released from the cell component can be detected.

One suitable system that allows the detection of released biomolecular inhibitor (or activator) is one in which fluorescence polarization of molecules in the milieu can be measured. The biomolecular inhibitor can have bound to it a fluorescent tag or label such as fluorescein or fluorescein attached to a linker. Assays for inhibition of the binding of the biomolecular inhibitor to the cell component can be done in microtiter plates to conveniently test a set of compounds at the same time. In such assays, a majority of the fluorescently labeled biomolecular inhibitor must bind to the protein in the absence of competitor compound to allow for the detection of small changes in the bound versus free probe population when a compound which is a competitor with a biomolecular inhibitor is added (B. A. Lynch, et al., Analytical Biochemistry 247:77-82 (1997)). If a compound competes with the biomolecular inhibitor for a binding site on the target cell component, then fluorescently labeled biomolecular inhibitor is released from the target cell component, lowering the polarization measured in the milieu.

In a further method for identifying one or more compounds that compete with a biomolecular inhibitor (or activator) for a binding site on a target cell component, the target cell component can be attached to a solid support, contacted with one or more compounds, and contacted with the biomolecular inhibitor. One or more washing steps can be employed to remove biomolecular inhibitor and compound not bound to the cell component. Either the biomolecular inhibitor bound to the target cell component or the compound bound to the target cell component can be measured. Detection of biomolecular inhibitor or compound bound to the cell compound can be facilitated by the use of a label on either molecule type, wherein the label can be, for instance, a radioactive isotope either incorporated into the molecule itself or attached as an adduct, streptavidin or biotin, a fluorescent label or a substrate for an enzyme that can produce from the substrate a colored or fluorescent product. An appropriate means of detection of the labeled biomolecular inhibitor or compound moiety of the biomolecular inhibitor-cell component complex or the compound-cell component complex can be applied. For example, a scintillation counter can be used to measure radioactivity. Radiolabeled streptavidin or biotin can be allowed to bind to biotin or streptavidin, respectively, and the resulting complexes detected in a scintillation counter. Alkaline phosphatase conjugated to streptavidin can be added to a biotin-labeled biomolecular inhibitor or compound. Detection and quantitation of a biotin-labeled complex can then be by addition of pNPP substrate of alkaline phosphatase and detection by spectrophotometry, of a product which absorbs UV light at a wavelength of 405 nm. A fluorescent label can also be used, in which case detection of fluorescent complexes can be by a fluorometer. Models are available that can read multiple samples, as in a microtiter plate.

For example, in one type of assay, the method for identifying compounds comprises attaching the target cell component to a solid support, contacting the biomolecular inhibitor with the target cell component under conditions suitable for binding of the biomolecular inhibitor to the cell component, removing unbound biomolecular inhibitor from the solid support, contacting one or more compounds (e.g., a mixture of compounds) with the cell component under conditions suitable for binding of the biomolecular inhibitor to the cell component, and testing for unbound biomolecular inhibitor released from the cell component, whereby if unbound biomolecular inhibitor is detected, one or more compounds that displace or compete with the biomolecular inhibitor for a particular site on the target cell component have been identified.

Other methods for identifying compounds that are competitive binders with the biomolecule for a target can employ adaptations of fluoresence polarization methods. See, for instance, Anal. Biochem. 253(2):210-218 (1997), Anal. Biochem. 249(1):29-36 (1997), BioTechniques 17(3):585-589 (1994) and Nature 373:254-256 (1995).

Those compounds that bind competitively to the target cell component can be considered to be drug candidates. Further appropriate testing can confirm that those compounds which bind competitively with biomolecular inhibitors (or activators) possess the same activity as seen in an intracellular test of the effect of the biomolecular inhibitor or activator upon the phenotype of cells. Derivatives of these compounds having modifications to confer improved solubility, stability, etc., can also be tested for a desired phenotypic effect.

Combining steps for testing the phenotypic effects of a biomolecule, as can be produced in an intracellular test, with steps for identifying compounds that compete with the biomolecule for sites on a target cell component, yields a method for identifying a compound which is a functional analog of a biomolecule which produces a phenotypic effect on a cell. These steps can be to test, for the phenotypic effect, either in culture or in an animal model, or in both, a cell which produces a biomolecule by regulable expression of an exogenous gene in the cell, and to identify, if the biomolecule caused the phenotypic effect, one or more compounds that compete with the biomolecule for binding to a target cell component. If a compound is found to compete with the biomolecule for binding to the target cell component, then the compound is a functional analog of a biomolecule which produces a phenotypic effect on the cell. Such a functional analog can cause qualitatively a similar effect on the cell, but to a similar degree, lesser degree or greater degree than the biomolecule.

A further embodiment of the invention combining general steps (1) and (2) is a method for determining whether a target component of a cell is essential to producing a phenotypic effect on the cell, comprising isolating the target component from the cell, identifying a biomolecular binder of the isolated target component of the cell, constructing a second cell comprising the target component and a regulable, exogenous gene encoding the biomolecular binder, and testing the second cell in culture for an altered phenotypic effect, upon production of the biomolecular binder in the second cell, whereby, if the second cell shows the altered phenotypic effect upon production of the bimolecular binder, then the target component of the first cell is essential to producing the phenotypic effect on the first cell.

The methods described herein are well suited to the identification of compounds that can inhibit the proliferation of the cells of infectious agents such as bacteria, fungi and the like. In addition, a procedure such as the one outlined below can be used in the identification of compounds to inhibit the proliferation of cancer cells. The two procedures described below further illustrate the use of the methods described herein and would provide proof of principle of these methods with a known target for anticancer therapy.

Mammalian dihydrofolate reductase (DHFR) is a proven target for anticancer therapy. Methotrexate (MTX) is one of many existing drugs that inhibit DHFR. It is widely used for anticancer chemotherapy.

NIH 3T3 is a mouse fibroblast cell line that is able to develop spontaneous transformed cells when cultured in low concentration (2%) of calf serum in molecular, cellular and developmental biology medium 402 (MCDB) (M. Chow and H. Rubin, Proc. Natl. Acad. Sci. USA 95(8):4550-4555 (1998)). The transformed cells, which can be selectively inhibited by MTX (Chow and Rubin), are isolated. Both the normal and transformed NIH3T3 cells are transfected with pTet-On plasmid (Clontech; Palo Alto, Calif.). Stable cell lines that express high levels of reverse tetracycline-controlled activator (rtTA) are isolated and characterized for their normal or transformed phenotype (Chow and Rubin).

The DHFR gene (GenBank Accession #L26316) from the NIH 3T3 cell line is amplified by reverse transcription-PCR (RT-PCR) using poly A⁺ RNA isolated from NIH 3T3 cells (Sambrook, J. et al., Molecular Cloning: A Laboratory Manual, 2nd edition, Cold Spring Harbor Laboratory Press, 1989). Active DHFR is expressed using the BacPAK Baculovirus Expression System (Clontech) or other appropriate systems. The expressed DHFR is purified and biotinylated and subjected to peptide binder identification as exemplified for bacterial proteins. The identified peptides are biochemically characterized for in vitro inhibition of DHFR activity. Peptides that inhibit DHFR are identified. A nucleic acid encoding each peptide can be cloned into a vector such as pGEX-4T2 (Pharmacia) to yield a vector which encodes a fusion polypeptide having the peptide fused to the N-terminus of GST. This can also be done by PCR amplification as exemplified herein for the peptide Pro-3. The fusion genes are cloned into plasmid pTRE (Clontech) for regulated expression. The constructed plasmid or the vector is cotransfected with pTK-Hyg into the stable NIH 3T3 cell line that expresses rtTA. The resulting cell lines, termed 3T3N-VITA (normal 3T3 cells that express rtTA and the DHFR inhibitory peptides), 3T3T-VITA (transformed 3T3 cells that express rtTA and the DHFR inhibitory peptides), or 3T3T-VITA control (transformed 3T3 cells that express rtTA and GST), are characterized for their normal or transformed phenotype (loss of contact inhibition, change in morphology, immortalization, etc.).

10²-10³ of 3T3T-VITA or 3T3T-VITA control cells are mixed with 10⁵ 3T3N-VITA and are grown in MCDB 402 medium with 10% calf serum at 37° C. for three days. Tetracycline is added to the medium to a final concentration of 0 to 1 μg/ml. In a control, 200 nM of MTX is added. The cultures are incubated for an additional eight days, and the number of foci formed are counted as described by M. Chow and H. Rubin, Proc. Natl. Acad. Sci. USA 95(8):4550-4555 (1998). Peptides that specifically inhibit foci formation of 3T3 transformed cells are identified.

A murine model of fibroblastoma (Kogerman, P. et al., Oncogene 15(12):1407-1416 (1997)) is used for evaluating the DHFR/peptide combination for identification of compounds for cancer therapy. Various amounts of 3T3T-VITA or 3T3T-VITA control cells (10³, 10⁴, 10⁵, 10⁶ cells) are injected subcutaneously into 5 groups (10 in each group) of athymic nude mice (4-6 weeks old, 18-22 g) to determine the minimal dose needed for development of fibroblastomas in all of the tested animals. Upon determination of the minimal tumorigenic dose, 6 groups of athymic nude mice (10 each) are injected subcutaneously (s.c.) with the minimal tumorigenic dose for 3T3T-VITA or 3T3T-VITA control cells to develop fibroblastoma. One week after injection, group 1 mice start receiving MTX s.c. at 2 mg/kg/day as positive control, group 2 to 5 start receiving 1, 2, 5, or 10 mg/kg/day of tetracycline, group 6 start receiving saline (vehicle) as control. Five weeks after the introduction of cells, all of the mice are sacrificed and tumors are removed from them. Tumor mass is measured and compared among the groups.

An effective peptide identified by these in vivo experiments can be used for screening libraries of compounds to identify those compounds that competitively bind to DHFR.

One mechanism of tumorigenesis is overexpression of proto-oncogenes such as Ha-ras (Reviewed by Suarez, H. G., Anticancer Research 9(5):1331-1343 (1989)). Compounds that inhibit the activities of the products of such proto-oncogenes can be used for cancer chemotherapy. What follows is a further illustration of the methods described herein, as applied to mammalian cells.

Transgenic mice that overexpress human Ha-ras have been produced. Such transgenic mice develop salivary and/or mammary adenocarcinomas (Nielsen, L. L. et al, In Vivo 8(5):1331-1343 (1994)). Secondary transgenic mice that express rtTA can be generated using the pTet-On plasmid from Clontech.

Human Ha-ras open reading frame cDNA (GenBank Accession #G00277) is amplified by RT-PCR using polyA⁺ RNA isolated from human mammary gland or other tissues. Active Ha-ras is expressed using the BacPAK Baculovirus Expression System (Clontech) or other appropriate systems. The expressed Ha-ras is purified and biotinylated and subjected to peptide binder identification as exemplified herein for bacterial proteins as target cell components. The identified peptides are biochemically characterized for in vitro inhibition of Ha-ras GTPase activity. Peptides that inhibit Ha-ras are cloned into plasmid pTRE (Clontech) for regulated expression as an N-terminal fusion of GST. Such constructs are used to generate tertiary transgenic mice using the secondary transgenic mice. Transgenic mice that are able to overexpress peptide genes are identified by Northern and Western analysis. Control mice that express GST are also identified.

Various doses of tetracycline are administered to the tertiary transgenic mice by s.c. or i.p. injection before or after tumor onset. Prevention or regression of tumors resulting from expression of the peptide genes are analyzed as described above for murine fibroblastoma.

Peptides found to be effective in in vivo experiments will be used to screen compounds that inhibit human Ha-ras activity for cancer therapy.

The method of the invention can be applied more generally to mammalian diseases caused by: (1) loss or gain of protein function, (2) over-expression or loss of regulation of protein activity. In each case the starting point is the identification of a putative protein target or metabolic pathway involved in the disease. The protocol can sometimes vary with the disease indication, depending on the availability of cell culture and animal model systems to study the disease. In all cases the process can deliver a validated target and assay combination to support the initiation of drug discovery.

Appropriate disease indications include, but are not limited to, Alzheimer's, arthritis, cancer, cardiovascular diseases, central nervous system disorders, diabetes, depression, hypertension, inflammation, obesity and pain.

Appropriate protein targets putatively linked to disease indications include, but are not limited to (1) the leptin protein, putatively linked to obesity and diabetes; (2) a mitogen-activated protein kinase putatively linked to arthritis, osteoporosis and atherosclerosis; (3) the interleukin-1 beta converting protein putatively linked to arthritis, asthma and inflammation; (4) the caspase proteins putatively linked to neurodegenerative diseases such as Alzheimer's, Parkinson's and stroke, and (5) the tumor necrosis factor protein putatively linked to obesity and diabetes. Appropriate protein targets include also, but are not limited to, enzymes catalyzing the following types of reactions: (1) oxido-reductases, (2) transferases, (3) hydrolases, (4) lyases, (5) isomerases, and (6) ligases.

The arachidonic acid pathway constitutes one of the main mechanisms for the production of pain and inflammation. The pathway produces different classes of end products, including the prostaglandins, thromboxane and leukotrienes. Prostaglandins, an end product of cyclooxygenase metabolism, modulate immune function, mediate vascular phases of inflammation and are potent vasodilators. The major therapeutic action of aspirin and other non-steroidal anti-inflammatory drugs (NSAIDs) is proposed to be inhibition of the enzyme cyclooxygenase (COX). Anti-inflammatory potencies of different NSAIDs have been shown to be proportional to their action as COX inhibitors. It has also been shown that COX inhibition produces toxic side effects such as erosive gastritis and renal toxicity. The knowledge base regarding the toxic side effects of COX inhibitors has been gained through years of monitoring human therapies and human suffering. Two kinds of COX enzymes are now known to exist, with inhibition of COX1 related to toxicity, and inhibition of COX2 related to reduction of inflammation. Thus, selective COX2 inhibition is a desirable characteristic of new anti-inflammatory drugs. The method of the invention can provide a route from identification of potential drug targets to validating these targets (for example, COX1 and COX2) as playing a role in disease (pain and inflammation) to an examination of the phenotype for the inhibition of one or both target isozymes without human suffering. Importantly, this information can be collected in vivo.

As an alternative strategy, the method of the invention can be used to define the phenotype of “genes of unknown function” obtained from various human genome sequencing projects or to assess the phenotype resulting from inhibition of one isozyme subtype or one member of a family of related protein targets.

The present invention is more specifically illustrated in the following examples, which are not intended to be limiting in any way.

EXAMPLES Example 1

Isolating a Peptide That Binds to and Inhibits E. coli Prolyl-tRNA Synthetase

Because of its well established genetic and expression systems, Escherichia coli was chosen for initial tests of the methods. Prolyl-tRNA synthetase (ProRS) catalyzes the attachment of proline to its cognate tRNA for protein biosynthesis. It is an enzyme of essential cellular function and is a good candidate for target validation.

In order to produce pure protein for peptide selection, the E. coli ProRS gene (Genbank accession number: M97858) was PCR amplified and cloned into the pET-20b vector (Novagen) using standard molecular cloning protocols (J. Sambrook, et al., Molecular Cloning: A Laboratory Manual, 2^(nd) edition, Cold Spring Harbor Laboratory Press, 1989). The overexpressed ProRS was purified sequentially on Q- and SP-Sepharose columns. The purified ProRS was biotinylated using EZ-link™ Sulfo-NHS-LC-Biotin from Pierce according to the instructions packaged with the biotinylation compound, captured onto streptavidin-agarose beads and used to select specific binding peptides from a peptide library displayed on coat protein III of M13 phages using a standard protocol (J. K. Scott and G. P. Smith, Science 249:386-390 (1990)). Thirteen clones from phages that were identified as having a high affinity to ProRS were sequenced. These clones share 4 different sequences, 3 of which are closely related (Table 1). See Example 4 of WO98/19162, published May 7, 1998.

TABLE 1 Peptide sequences with high binding affinity to E. coli ProRS SEQ ID NO: Peptide Sequence # of Phage Isolated 1 SRDWGFWDWGVDRSR 5 2 SRDWGFWRLPESMASR 3 3 SREWHFWRDYNPTSR 4 4 SSERGSGDRGEKGSR 1

The peptide having sequence number 3 was synthesized and tested for inhibition of tRNA charging activity of E. coli ProRS. This peptide inhibits E. coli ProRS aminoacylation activity, demonstrates competitive inhibition with both proline and ATP, and exhibits a K_(i) of 300 nM. This peptide is called Pro-3 (also, Pro3).

Example 2

Regulated Expression of Pro-3 in E. coli Causes Cessation of Cell Growth

Regulated intracellular expression of the Pro-3 peptide in E. coli was achieved by fusing an oligonucleotide encoding the peptide to the 5′ end of a gene encoding a glutathione S-transferase (GST) protein. To generate the peptide expression construct, the Pro-3/GST fusion gene was PCR amplified using a combination of the Pro3, Pro3/GST, and GST/R primers as illustrated in FIG. 1. Primers Pro3 and Pro3/GST encode the Pro-3 peptide sequence; the latter anneals to the 5′ end of the GST gene on plasmid pGEX-4T2 (Pharmacia). A 4-amino acid linker was introduced between the Pro-3 peptide and GST for flexibility. The PCR product was further amplified with primers Pro3(Kpn) and GST/R(Bam), digested with KpnI and BamHI restriction endonucleases, and ligated to the KpnI/BamHI sites of the expression vector pPROTet (Clontech, Palo Alto, Calif.) using standard cloning protocols. pPROTet uses the P_(L) promoter of phage lambda combined with the Tet operator of the Tn10 tetracycline resistance operon to direct the regulated expression of the cloned gene (Clontech, PROTM Bacterial Expression System User Manual, PT3161-1, Version PR7Y629). The ligated DNA was then used to transform DH5αPRO (Clontech), an E. coli strain expressing the Tet repressor. Clone pC³844 was sequenced and identified as containing the Pro-3/GST fusion gene. The linker between the Pro3 peptide and GST is Glu-Gly-Gly-Gly (SEQ ID NO:46). pC³844 was also transformed into the E. coli strain JM109/pSC, which is JM109 harboring a plasmid expressing Tet repressor that was isolated from BL21PRO (Clontech). The resulting E. coli strain is called JM109/pSC/pC³844.

Table 2

Oligonucleotides Used to Generate Pro-3/GST Expression Construct

Pro3: 5′CCAACAACATATGTCCCGTGAATGGCACTTCTGGCGTGACTAC  (SEQ ID NO:5)

Pro3/GST: 5′TTCTGGCGTGACTACAACCCGACCTCCCGTGGGGGTGGAGGCATGTCCCCTATACTA  (SEQ ID NO:6)

GST/R: 5′AGTTGAATTCTTAATCCGATTTTGGAGGATGG  (SEQ ID NO:7)

Pro(Kpn): 5′CAAGGTACCCATGTCCCGTGAATGGCAC  (SEQ ID NO:8)

GST/R(Bam): 5′CGCGGATCCTTAATCCGATTTTGGAGGATGG  (SEQ ID NO:9)

To test the expression of the Pro-3/GST fusion gene, overnight cultures of DH5αPRO harboring pC³844 or pPROtet-LacZ (Clontech), a control expression plasmid with the same backbone as pC³844 and a LacZ gene for expression, were inoculated into fresh LB broth containing 34 μg/mL chloramphenicol (an antibiotic for maintaining the expression plasmid), and 50 μg/mL spectinomycin (an antibiotic for maintaining the Tet repressor gene). Expression was induced by addition of anhydrotetracycline to 0, 10, 100, or 500 ng/mL, when the bacterial culture reached an OD₆₀₀ of 0.5. Anhydrotetracycline is a derivative of tetracycline that acts as a potent inducer of the TetO/R system and is less toxic to E. coli than tetracycline (T. Lederer, et al., Biochemistry, 35:7439-7446 (1996); B. Oliva, et al., Antimicrob. Agents Chemother. 36:913-919 (1992)). After 2.5 hours of induction by exposure to anhydrotetracycline, cells were pelleted from 1 mL culture and lysed by boiling in 100 μL of SDS-PAGE sample buffer. The samples were examined with SDS-PAGE; gels were stained with Coomassie Blue (FIG. 2). The results demonstrated that while basal level expression is undetectable, the Pro-3/GST fusion protein was produced to a significant level upon anhydrotetracycline induction, a level comparable to β-galactosidase production from lacZ. The reduced amount of total protein in the expression samples of Pro-3/GST but not the LacZ control correlates with the growth inhibition observed during the induction of Pro-3/GST (see below).

The expressed Pro-3/GST fusion protein was purified on a glutathione-agarose affinity column according to a procedure provided by Pharmacia (see, e.g., procedures manual from Pharmacia P-L Biochemicals, Inc.: GST Gene Fusion System, regarding use of pGEX expression vectors and glutathione-S-transferase fusion proteins, 1993). The N-terminal sequence of the purified protein was determined by Edman cycles and confirmed to have the expected Pro-3 peptide sequence. The purified Pro-3/GST was also confirmed to inhibit E. coli ProRS activity, with a K_(i) of 180 nM, similar to that of the Pro-3 peptide.

That expression of Pro-3/GST, but not of GST alone inhibits E. coli growth, was demonstrated by the following experiment. Overnight cultures of JM109/pSC cells harboring pC³844 (JM109/pSC/pC³844), or pPROTet-GST (same as pC³844 without the Pro-3 peptide sequence) were diluted 100-fold in fresh LB medium containing 34 μg/mL chloramphenicol and 50 μg/mL spectinomycin. After 2 hours growth at 37° C., 100 μL aliquots of each of these bacterial cultures were transferred to wells of a 96-well microtiter plate, with or without 10 ng of anhydrotetracycline dissolved in 1 μL ethanol. The growth of bacterial cultures, which were constantly under agitation, was monitored with a plate reader (SpectraMAX-250, Molecular Devices). The results depicted in FIG. 3A and FIG. 3B demonstrate that E. coli growth was specifically inhibited by expression of the Pro-3 peptide.

Example 3

In vivo Expression of Pro-3 Peptide to Cure a Lethal Infection

Intracellularly expressed Pro-3 was expected to bind to and inhibit a specific essential cellular target in a manner similar to that of an antimicrobial drug. Pro-3 was expected to cure the infection when its expression was induced in an animal model of bacterial infection. An established animal infection model was used to test this concept (C. O. Onyeji et al., Antimicrob. Agents Chemother. 38:1112-1117 (1994)).

Eight groups of CD-1 female mice (5 mice per group, Charles River Laboratories; Wilmington, Mass.) weighing 20-24 g were used in this experiment. The inoculum was prepared from E. coli JM109/pSC/pC³844 which was cultured at 37° C. for 17 hr in Luria-Bertani broth containing spectinomycin and chloramphenicol, and then 100 μl of the culture was diluted to 1 ml with medium for reading OD at 600 nm (0.2403, the medium as blank). The turbidity of a 0.5 McFarland standard is equivalent to OD₆₀₀ 0.1, or 10⁸ cfu/ml. Then 3×10⁹ cfu (colony-forming units) of E. coli JM109/pSC/pC³844 (1.25 ml) from the overnight culture were diluted to 15 ml with 0.01 M phosphate buffered saline (Sigma P-0261) containing 8% hog gastric mucin (Sigma M-2378), 100 μg/ml spectinomycin and 68 μg/ml chloramphenicol. Each mouse of groups 1 through 4 was injected with 0.5 ml of the inoculum intraperitoneally (i.p.), equivalent to 1×10⁸ cfu/mouse (lethal dose).

Groups 5 through 8 served as vector control. The control inoculum was prepared from E. coli pPROTet-GST whose vector carries a gene encoding glutathione S-transferase, but no Pro-3 peptide. E. coli pPROTet-GST was cultured at 37° C. for 17 hr in Luria-Bertani broth containing spectinomycin and chloramphenicol, and then 100 μl of the culture were diluted to 1 ml with the medium for reading the OD at 600 nm (0.2858, the medium as blank). Then 3×10⁹ cfu (1.05 ml) from the overnight culture were diluted to 15 ml with 0.01 M phosphate buffered saline containing 8% hog gastric mucin. Each mouse of groups 5 through 8 was injected (i.p.) with E. coli pPROTet-GST inocula equivalent to 1×10⁸ cfu/mouse (lethal dose).

One and four hours after the inoculation, groups 1 and 5 animals received a saline injection i.p. at 10 ml/kg; groups 2, 3 and 4 animals received i.p. injections of anhydrotetracycline at 2, 1 and 0.5 mg/kg (diluted in saline), respectively; groups 6, 7 and 8 received i.p. injections of anhydrotetracycline at 2, 1 and 0.5 mg/kg, respectively. The injection volume for all the animals was 10 ml/kg.

The data summarized in Table 3 demonstrate that inhibition of E. coli ProRS activity by in vivo intracellular expression of Pro-3 peptide cures a lethal infection in the mouse model.

TABLE 3 # Survival Group of mice Inoculation Treatment (7 days) 1 5 E. coli JM109/pSC/pC³844 Saline 10 ml/ 0/5 10⁸ cfu/mouse, i.p. kg, i.p. x2 2 5 E. coli JM109/pSC/pC³844 Anhydrote- 5/5 10⁸ cfu/mouse, i.p. tracycline 2 mg/kg, i.p. x2 3 5 E. coli JM 109/pSC/pC³844 Anhydrote- 5/5 10⁸ cfu/mouse, i.p. tracycline 1 mg/kg, i.p. x2 4 5 E. coli JM109/pSC/pC³844 Anhydrote- 5/5 10⁸ cfu/mouse, i.p. tracycline 0.5 mg/kg, i.p. x2 5 5 E. coli pPROTet-GST, Saline 10 ml/ 0/5 10⁸ cfu/mouse, i.p. kg, i.p. x2 6 5 E. coli pPRoTet-GST; Anhydrote- 0/5 10⁸ cfu/mouse, i.p. tracycline 2 mg/kg, i.p. x2 7 5 E. coli pPROTet-GST, Anhydrote- 0/5 10⁸ cfu/mouse, i.p. tracycline 1 mg/kg, i.p. x2 8 5 E. coli pPROTet-GST, Anhydrote- 0/5 10⁸ cfu/mouse, i.p. tracycline 0.5 mg/kg, i.p. x2

Example 4

Specific Inhibition of ProRS: Examination of Intracellular tRNA Charging Levels

The levels of aminoacylated tRNAs in E. coli cells either with or without expression of Pro-3 peptide were examined in order to confirm that E. coli growth inhibition was caused by inhibition of ProRS activity. For this purpose, 400 ml of fresh LB broth containing 34 μg/ml chloramphenicol and 50 μg/ml spectinomycin were inoculated with 4 ml JM109/pSC/pC³844 overnight culture and grown at 37° C. to an OD₆₀₀ of 0.3. The bacterial culture was split into two 200 ml subcultures. To one of them anhydrotetracycline was added to final concentration of 500 ng/ml. After an additional 35 minutes, the bacterial cells were harvested and the level of charged tRNA^(Pro), tRNA^(Met) and tRNA^(Phe) were determined following the protocol described by Folk and Berg (W. R. Folk and P. Berg, Journal of Bacteriology 102:204-212 (1970)). The results as summarized in Table 4 indicate that expression of Pro-3 specifically inhibits charging of tRNA^(Pro).

TABLE 4 Charged tRNA aaRS −Inducer +Inducer ProRS  70% 23% MetRS 108% 96% PheRS 104% 98%

Example 5

Functional Complementation of Pro-3 Peptide Inhibition by Expression of a Heterologous ProRS Gene

If ProRS is the primary intracellular target of the Pro-3 peptide, E. coli growth inhibition by Pro-3 peptide should be relieved by functional complementation with a heterologous ProRS. It was found that Pro-3 does not inhibit S. aureus ProRS enzyme activity, which efficiently charges E. coli tRNA^(Pro). The S. aureus ProRS gene (WO 97/26343; EP 785272) was amplified with oligonucleotides S.PRS/XhoI-5′ (5′AAT CCG CTC GAG GAT TAT TGC TAT TGG TGC C) (SEQ ID NO:10) and S.PRS/Hind-3′ (5′AAT CGT AAG CTT TTA TTT TAA GTT ATC ATA TTT) (SEQ ID NO:11), digested with Xho I/Hind III restriction endonucleases, and cloned into Xho I/Hind III sites in pACYC177. The cloned S. aureus ProRS gene carries its own promoter and ribosome binding site, and is in the same orientation as the disrupted kanamycin resistance gene in the vector for efficient expression. Either the S. aureus ProRS expression construct or the pACYC177 vector alone was transformed into DH5αPRO /pC³844. The growth of the resulting E. coli strains was followed in the presence or absence of 200 ng/ml anhydrotetracycline. As depicted in FIG. 4A and FIG. 4B, with the S. aureus ProRS expression construct, E. coli cell growth was no longer inhibited by expression of the Pro-3 peptide. The results proved that the growth inhibition by the Pro-3 peptide is specifically caused by inhibition of ProRS activity.

Example 6

Development of Assay for E. coli ProRS and Pro3 Peptide

A non-radioactive, homogeneous, sensitive Fluorescent Polarization (FP) assay has been developed for E. coli ProRS and Pro3 peptide. FP is a radiometric detection method which is capable of discriminating between free and bound states of a fluorescently labeled tracer based on differences in the rotation rates of the two states. The E. coli ProRS FP binding assay involves the incubation of fluorescently labeled Pro3 peptide (Pro3-F) with purified E. coli ProRS. In the bound state, Pro3-F is bound to E. coli ProRS and an increase in polarization signal is detected using a FP detection system. In the free state, Pro3-F is not bound to E. coli ProRS and there is a decrease in polarization signal.

See Example 1 for cloning and purification of E. coli ProRS. See Example 1 (SEQ ID NO:3) for a description of Pro3 peptide. Fluorescently labeled Pro3 peptide (Pro3-F) was synthesized and purified by SynPep Corp, Dublin, Calif. Its sequence is shown below.

 NH2-SREWHFWRDYNPTSRGGK(FITC)-CO-amide  (SEQ ID NO:12)

A 96-well plate (Costar cat #3915) was blocked for 1 hour at room temperature with 150 μL/well 2 mg/ml BSA (Fisher Biotech cat #BP1600-100) in 0.1 M NaHCO_(3.) The plate was then washed manually three times with 150 μL/well of TBST (10 mM Tris-HCl pH 8.0, 150 mM NaCl, 0.05% Tween 20). To each well, 10 uL of 10% DMSO/TBST or compound (0.0001-100 μM) in 10% DMSO/TBST was incubated for 20 minutes at room temperature with 50 μL of 2.36 μM E. coli ProRS in TBST (10 mM Tris-HCI pH 8.0, 150 mM NaCl, 0.05% Tween 20). Following the pre-incubation, 40 μL of 0.391 μM Pro3-F in TBST was added to each well, mixed and incubated at room temperature for 60 minutes. The plate was then read in an LJL Analyst (LJL Biosystems, Sunnyvale, Calif.) in fluorescent polarization mode.

To assess the ability of the E. coli ProRS/Pro3-F FP binding assay to detect μM and nM inhibitors, three compounds were tested. CB-16914 and CB-118831 are known inhibitors of E. coli ProRS (nM and μM inhibitors, respectively) and CB-680 was used as a negative control. As shown in FIG. 5, both CB-16914 and CB-118831 were shown to inhibit Pro3-F binding whereas CB-680 had no effect.

FIG. 5 shows fluorescence polarization binding assay inhibition curves for CB-16914 (0.0001-10 μM), CB-118831 (0.046-100 μM) and CB-680 (0.0001-10 μM) with 1.18 μM E. coli ProRS and 0.625 μM Pro3-F.

Example 7

S. aureus Expression Systems

E. coli/Bacillus shuttle expression vectors pWH353 and pWH354 were obtained from Professor Wolfgang Hillen (Mikrobielle Genetik, Universität Tübingen, Tübingen, Germany; see M. Geissendörfer and W. Hillen, Appl. Microbiol Biotechnol., 33:657-663 (1990); DE 3934454). These expression vectors carry a Tn10 tet repressor gene. They also contain synthetic promoters with one or two Tet repressor binding sites that are optimized for inducible expression in B. subtilis. These inducible promoters direct the expression of CAT (chloramphenicol acetyltransferase).

pWH353 and pWH354 were transformed into S. aureus RN4220 cells by electroporation (S. Schenk and R. A. Laddaga, CMS Microbiology Letters, 94:133-138 (1992)). The transformants were tested for inducible expression by growing in LB broth containing 30 μg/ml kanamycin. After the OD₆₀₀ reached 0.5, the cultures were split and tetracycline was added to one set of the cultures to a final concentration of 0.5 μg/ml. The cultures were maintained with aeration for 3 hours at 37° C. The S. aureus cells were then pelleted and resuspended in 80 mM Tris-HCl, pH 7.4 containing 200 μg/ml lysostaphin, incubated at 37° C. for 5 minutes, frozen and thawed twice on dry ice/ethanol and 37° C. water bath. The samples were then sonicated twice and centrifuged at 14,000 g for 10 minutes. The supernatants were collected and subjected to electrophoretic analysis on an 18% SDS-polyacrylamide gel stained with Coomassie blue (FIG. 6). The CAT activities in these samples were determined (Frankel, A. D. et al., Proc. Natl. Acad. Sci. USA 86:7397-7401 (1989)) and summarized in Table 5.

TABLE 5 Inducible expression of CAT activity in S. aureus Plasmid Induction Relative CAT Activity No Plasmid − ND No Plasmid + ND pWH353 − 41 pWH353 + >4110 pWH354 − 1 pWH354 + 350 ND: Not detectable

Example 8

Identification of Small Peptides that Specifically Bind to and Inhibit S. aureus Methionyl-tRNA Synthetase (MetRS)

The S. aureus MetRS gene has been cloned (EP 785269; WO 97/26350). The gene was PCR amplified and cloned into pGEX-4T2 (Pharmacia) for expression as a GST fusion using standard molecular cloning protocols. The expressed GST-S. aureus MetRS was purified on a glutathione agarose column and the GST moiety was removed by thrombin cleavage. The resulting nonfusion S. aureus MetRS was then biotinylated using EZ-Link™ Sulfo-NHS-LC-Biotin from Pierce according to the instructions enclosed with the biotinylation compound. The biotinylated S. aureus MetRS was used for selecting binding peptides from a 12-mer peptide library displayed on M13 phages (New England Biolabs). After 4 rounds of selection, 12 phage clones were isolated and sequenced. Out of these 12 clones, 11 yielded 4 different sequences as summarized in Table 6. Peptides of sequence JT01 and JT02 were synthesized. JT01 was tested for inhibition of S. aureus MetRS activity.

The activity of S. aureus MetRS was monitored by the aminoacylation reaction. The enzyme was diluted in 50 mM HEPES, pH 7.5, 5 mM DTT and 0.01% BSA. A typical 50 μL reaction mixture contained 30 mM HEPES, pH 7.5, 10 mM MgCl₂, 30 mM KCl, 90 μM E. coli tRNA, 2 mM ATP and 5 μM [³H]-methionine (10 Ci/mmol). The reaction was initiated by addition of enzyme at 25° C. At 4 different timepoints, 10 μL of the reaction was quenched into 150 μL cold 5% TCA on a Millipore filter plate. The filter plate is then washed three times with cold 5% TCA followed by water and then ethanol. The filter plate was dried before addition of 100 μL Packard Microscint-20 and counted on a Packard Top-Count microplate scintillation counter.

Inhibition studies were performed for the peptides against two of the substrates, ATP and methionine. This was done at varying concentrations of peptides by holding the concentration of one substrate at its K_(m) value (1.5 mM for ATP and 5 μM for methionine), while varying the other substrate concentration around its K_(m) value. The initial velocity data obtained were then analyzed using GraFit.

The results as depicted in FIG. 7 indicate that peptide JT01 is a competitive inhibitor of S. aureus MetRS. The K_(i)'s for JT01 are 138 nM (methionine) and 13 nM (ATP). The K_(i)'s for JT02 are 1.7 μM (methionine) and 0.5 μM (ATP) and 13 nM (ATP).

TABLE 6 Peptide sequences identified from phage panning Number of Phage Peptide Peptide Sequence SEQ ID NO: Isolated JT01 DPNTWQLRWPMH 13 7 JT02 MWDLPYIWSRPV 14 2 JT03 ADTLNWYYYASW 15 1 JT04 ANNLSTMKKLKQ 16 1

Example 9

Development of Assay for S. aureus MetRS and Met1 Peptide

A non-radioactive, homogeneous, sensitive fluorescent polarization (FP) assay has been developed for Staphylococcus aureus MetRS (Sa MetRS) and Met1 peptide. FP is a radiometric detection method which is capable of discriminating between free and bound states of fluorescently labeled tracer based on differences in the rotation rates of the two states. The Sa MetRS FP binding assay involves the incubation of fluorescently labeled Met1 peptide (Met1-F) with purified Sa MetRS. In the bound state, Met1-F is bound to Sa MetRS and an increase in polarization signal is detected using a FP detection system. In the free state, Met1-F is not bound to Sa MetRS and there is a decrease in polarization signal.

See Example 8 for cloning and purification of SaMetRS. Sa MetRS GST fusion protein was cleaved with thrombin prior to these studies. Fluorescently labeled Met1 peptide (Met1-F) was synthesized and purified by SynPep Corp, Dublin Calif. Its sequence is shown below.

NH₂-DPNTWQLRWPMHGGK(FITC)-CO-amide  (SEQ ID NO:17)

A 96-well plate (Costar cat #3915) was blocked for 1 hour at room temperature with 150 μL/well 2 mg/ml bovine serum albumin (BSA) (Fisher Biotech cat #BP1600-100) in 0.1 M NaHCO₃. The plate was then washed manually three times with 150 μL/well of TBS (10 mM Tris-HCl pH 8.0, 150 mM NaCl). To each well, 20 μL of 5% DMSO/TBS or compound (0.0001-100 μM) in 5% DMSO)/TBS was incubated for 20 minutes at room temperature with 80 μL of 0.025 μM Sa MetRS and 0.00625 μM Met1-F in reaction buffer (87.5 mM HEPES pH 7.5, 25 mM MgCl₂, 25 mM KCl, 25 μg/ml BSA, 5 mM DTT). The plate was then read in an LJL Analyst (LJL Biosystems, Sunnyvale Calif.) in fluorescent polarization mode.

To assess the ability of the Sa MetRS/Met1-F FP binding assay to detect μM and nM inhibitors, seven compounds were tested as well as the unlabeled Met-1 peptide. The seven compounds are known inhibitors (IC₅₀'s in nM to μM range) of Sa MetRS functional charging assay. As shown in Table 7, six of the seven inhibitors showed similar IC₅₀'s (within 3-4×) in the FP binding assay as compared to the functional assay. The FP assay was unable to detect CB-125552 which may be due to (1) poor compound solubility or (2) compound binding at a site on Sa MetRS different from the site bound by the Met-1 peptide.

TABLE 7 IC₅₀ values for known Sa MetRS inhibitors: Comparison between the functional charging assay and the fluoresence polarization binding assay Mechanism IC₅₀ (μM) Solu- of Inhibition Charging FP Binding Inhibitor ID bility Methionine ATP Assay Assay CB-088887 >100 ND ND 26 60% remaining activity @ 100 μM CB-098944 ND Competitive Com- 3.1 9 petitive CB-114208 33 Competitive Com- 0.3 0.7 petitive CB-125265 ND Mixed Mixed 0.6 2 CB-125552 11 ND ND 13.4 >100 CB-126854 >100 ND ND 4.1 5 MET-1 >100 Competitive Com- 0.2 0.7 peptide petitive CB-00231 >100 Slow tight 0.04 0.16 binding inhibitor ND = Not Determined

The FP assay for Sa MetRS inhibition has been evaluated to see if it is amenable for high throughput screening (HTS) for drug discovery. The FP assay for Sa MetRS has been reduced to 96-well format and has been automated which is necessary for HTS. FIG. 8 shows results of an experiment on a control plate that was run to assess the performance of the FP HTS assay. Sa MetRS was at 20 nM final concentration; Met1-F was at 5 nM final concentration. Wells 1-6 contained Met-1F alone, wells 7-8 contained Sa MetRS alone, wells 9-18 and 19-28 contained Met1-F from 640 nM to 1.25 μM, and wells 29-96 contained 1% DMSO. The mP range between free Met1-F and Met1-F bound by Sa MetRS was 141 mP. The IC₅₀ for Met1 was tested on the plate in duplicate and the results were consistent with previously determined values. The signal to noise for the plate assay (S/N) was 7:1 which is acceptable for HTS.

TABLE 8 Plate Statistics mean mP 244 std dev 9 free peptide 103 mP range 141 CB-000231 245 ± 17 IC₅₀ (μM) S/N 7/1

Example 10

Construction of Staphylococcus aureus Strains that Constitutively Express the Tet Repressor, TetR

Construction of pCL84(ΔTc, ΔH3)

pCL84 is a S. aureus chromosome integration plasmid (Lee, C. Y. et al., Gene 103:101-105, 1991) and was used for integration of the tet repressor gene into the S. aureus chromosome. pCL84 carries a tetracycline resistance gene (tet(K))that is derived from plasmid pT181 (Guay, G. G. et al., Plasmid 30:163-166, 1993). Since this tetracycline resistance gene encodes a protein that acts as a tetracycline efflux pump, which is not desirable for a tetracycline inducible expression system, the following steps were taken to eliminate this gene from pCL84. Plasmid construction steps are diagramed in FIG. 9. pCL84 was digested with restriction endonuclease HindIII, and the resulting HindIII overhangs were blunted using T4 DNA polymerase. The DNA sample was then subjected to agarose gel electrophoresis to separate the fragments. The “vector” portion (carrying the origin of replication, the gene encoding spectinomycin resistance, and attP) was purified using GeneClean (Bio 101), ligated using T4 DNA ligase, and transformed into E. coli NovaBlue cells (Novagen). Plasmids isolated from three of the resulting transformants were tested by restriction endonuclease HindIII or SalI digestion, and all three were confirmed to be pCL84 devoid of the Tc^(R) gene. One of these three plasmids was named pCL84(ΔTc, ΔH3) and was used in further procedures described below.

Construction of an ermC and tetR Transcription Fusion Gene in pCL84(ΔTc,ΔH3)

The ermC gene, including its promoter, was amplified from plasmid pE194 (ATCC Number 68359) using Taq DNA polymerase with oligonucleotide primers ermC-5′ (5′acgggtcgactcatatcttttattcaataatcg; SEQ ID NO:18) and ermC-3′ (5′ccggaaagcttacttattaaataatttatagc; SEQ ID NO:19). The tetR gene was amplified from plasmid pWH354 (DE 3934454 A1; Geissendörfer, M. and W. Hillen, Appl. Microbiol. Biotechnol. 33:657-663, 1990) using Taq DNA polymerase with primers tetR-5′ (5′taagtaagcttaaggaggaattaatgatgtctag; SEQ ID NO:20) and tetR-3′ (5′acgggtcgacttaagacccactttcacatttaag; SEQ ID NO:21). The amplified tetR gene carried a synthetic ribosomal binding site. These two PCR products were purified with the Wizard PCR Preparation Purification Kit (Promega), digested with HindIII restriction endonuclease, and ligated together with T4 DNA ligase. The resulting ligated DNA was then used as the template for PCR amplification using Taq DNA polymerase with primers ermC-5′ and TetR-3′. The PCR product was purified with the Wizard PCR Preparation Purification Kit (Promega), digested with SalI restrictionendonuclease to make SalI ends, and then further purified with Wizard PCR Preparation Purification Kit (Promega). The purified ermC-tetR fusion gene was then ligated to SalI-digested, shrimp alkaline phosphatase (SAP) treated, and gel purified pCL84(ΔTc,ΔH3). The ligated DNA was transformed into E. coli NovaBlue cells (Novagen). Fifty-five transformants were subjected to colony PCR screening using primers ermC-5′ and tetR-3′. Two clones, pCL84t #16, and #19, were identified as containing the ermC-tetR fusion gene. Plasmid DNA was purified from these two clones and the tetR gene sequence in clone #16 was confirmed by DNA sequencing. The ErmC gene provides not only a selectable phenotypic marker (erythromycin resistance), but also a constitutive promoter for the TetR gene.

Integration of ermC-tetR Fusion Gene into the S. aureus Chromosome

Plasmid DNA isolated from clone #16 was transformed into S. aureus CYL316 cells by electroporation (Schenk, S. and R. A. Laddaga, FEMS Microbiology Letters 94:133-138, 1992). The transformation was spread on LB plates containing 10 μg/ml of erythromycin. Six transformants were isolated and it was confirmed by a lipase assay (Lee, C. Y. et al., Gene 103:101-105, 1991) that ermC-tetR was integrated into the phage L54a integration site on the chromosome in all six. The resulting clones were designated strain CYL316t. The expected structure was confirmed by PCR experiments.

Confirmation of tetR Expression in the CYL316t S. aureus Strain

Overnight cultures of E. coli DH5αPRO, and cultures of S. aureus RN4220 (Novick, R. P. et al., Methods in Enzymology 204:587-637, 1991) carrying pWH354, S. aureus strain CYL316t, or CYL316:pCL84 were diluted 60-fold in 2 tubes of fresh LB containing 50 μg/ml spectinomycin (for DH5αPRO), 25 μg/ml kanamycin (for RN4220/pWH354), 10 μg/ml erythromycin (for CYL316t), or 3 μg/ml tetracycline (for CYL316:pCL84) and grown at 37° C. At 2.5 hours, anhydrotetracycline was added to one set of cultures to 200 ng/ml. The cells were grown for an additional 3 hours. The cells were pelleted from 3 ml of culture and resuspended in 200 μL of 1×PBS containing 50 μg/ml lysostaphin. After incubating at 4° C. for 30 minutes, the cell suspensions were subjected to three freeze-thaw cycles between a dry-ice/ethanol bath and a 37° C. water bath, followed by a 15-second sonication with a microtip. The cell lysates were then boiled in SDS-PAGE sample buffer and subjected to electrophoresis on a 12.5% SDS-polyacrylamide gel. The separated proteins were then blotted to a nitrocellulose membrane and analyzed by Western blot using an anti-Tet repressor protein antibody (kindly provided by Prof. Wolfgang Hillen). The results confirmed that tetR is constitutively expressed in S. aureus strain CYL316t.

Construction of a Staphylococcus aureus Strain that Constitutively Expresses tetR as Well as tetM

An EcoRI fragment containing the tet(M) gene of Tn916 was isolated from plasmid pVB101, a construct similar to pVB201 (Burdett, V., J. Bacteriol. 175:7209-7265, 1993). The tet(M) gene fragment was cloned into the EcoRI site of pCL84t, resulting in plasmid pCL84tt. pCL84tt was integrated into the S. aureus chromosome in CYL316, resulting in strain CYL316tt. CYL316tt can grow in LB medium containing 10 μg/ml tetracycline.

Example 11

Construction of Expression Plasmids

The steps in plasmid construction are diagramed in FIG. 10.

A. pWH353 and pWH354 Derivatives

E. coli/Bacillus subtilis shuttle expression vectors pWH353 and pWH354 were obtained from Professor Wolfgang Hillen (Mikrobielle Genetik, Universität Tübingen, Tübingen, Germany; DE 3934454 A1; Geissendörfer, M. and W. Hillen, Appl. Microbiol. Biotechnol. 33:657-663, 1990). These expression vectors carry a Tn10 tetR gene encoding Tet repressor. They also contain synthetic promoters with one or two Tet repressor binding sites that are optimized for inducible expression in B. subtilis. These inducible promoters direct the expression of a chloramphenicol acetyltransferase (CAT) gene in each case.

The CAT gene in pWH353 or pWH354 was replaced with a GST gene. The GST gene was PCR amplified with Taq DNA polymerase using pGEX-4T-2 as the template and the combination of oligonucleotide primers 2a-5′-GST (5′CTC GGT ACC GAG CTA AAA TTC GGA GGC ATA TCA AAT GAG CTC TGG) (SEQ ID NO:22), 2b-5′-GST (5′GGC ATA TCA AAT GAG CTC TGG AGG TGG AGG CAT GTC CCC TAT AC) (SEQ ID NO:23), and 3′-GST-AvrII (5′AGG CCT AGG TTA ATC CGA TTT TGG AGG ATG G) (SEQ ID NO:24) as the primers. The PCR fragment was cloned into pT7Blue(T) vector (Novagen). The GST gene in the resulting pT7Blue(T) vector was then excised by KpnI and StuI double-endonuclease digestion and further cloned into the KpnI/StuI sites of pWH353 and pWH354, resulting in pWH353-2G, and pWH354-2G. A synthetic gene encoding peptide JT101 was obtained by annealling oligonucleotides M1.Sac.a (5′CTG ATC CGA ATA CGT GGC AGT TGC GGT GGC CTA TGC ATA GCT) (SEQ ID NO:25) and M1.Sac.b (5′ATG CAT AGG CCA CCG CAA CTG CCA CGT ATT CGG ATC AGA GCT) (SEQ ID NO:26) and cloned into the Sacd site of pWH353-2G and pWH354-2G. Clones pWH353-2G-Ml and pWH354-2G-M1 were identified that contained the M1-GST fusion gene.

B. pWH353(ΔtetR) and pWH354(ΔtetR) and Their Derivatives

Plasmids pWH353, pWH354 and pWH354-2G were each digested with restriction endonucleases SpeI and BstEII, which excise a DNA fragment encompassing the tetR gene in these two plasmids. The overhangs of the resulting DNA fragments were blunted using T4 DNA polymerase. The “vector” portion of each of these two digestions (the remainder of the plasmid without tetR) was gel purified, self-ligated using T4 DNA ligase, and was then transformed into E. coli NovaBlue cells (Novagen). Clones carrying pWH353 or pWH354 or pWH354-2G deleted for the tetR gene were identified by PCR colony screening, and were designated pWH353(ΔtetR), pWH354(ΔtetR) and pWH354-2G(ΔtetR), respectively.

C. pC³859 and its Derivatives

The following four oligonucleotides were chemically synthesized:

Tet-2OF: 5′tcgagttcatgaaaaactaaaaaaaatattgacatccctatcagtgat  (SEQ ID NO:27)

Tet2/3OF: 5′agagataattaaaataatccctatcagtgatagagagcttgcatg  (SEQ ID NO:28)

Tet2/3OR: 5′caagctctctatcactgatagggattatt  (SEQ ID NO:29)

Tet2OR: 5′ttaattatctctatcactgatagggatgtcaatattttttttagtttttcatgaac  (SEQ ID NO:30)

Oligonucleotides Tet-2OR and Tet-2/3OF were phosphorylated using T4 polynucleotide kinase. Equal molar amounts of the phosphorylated Tet-2OR, Tet-2/3OF, and unphosphorylated Tet-2OF, Tet-2/3OR were mixed together, heated to 85° C. for 5 minutes and cooled down to room temperature gradually in a 2-hour period. The annealed oligonucleotides, called Tet-2O, comprised a synthetic promoter with two tet2O operator sequences that are derived from Tn10.

pWH354(ΔtetR) was digested with restriction endonucleases XhoI and SphI, which excised the promoter/operator that controlled CAT expression. The digested DNA was separated on a 1% agarose gel. The DNA fragment of the vector portion was purified and ligated with Tet-2O DNA fragment. The ligation was transformed into DH5αPRO (Clontech). A clone of the desired recombination was identified and designated pC³859.

The CAT gene open reading frame in pC³859 was substituted with the GST gene amplified from plasmid pGEX-4T-2 (Pharmacia), green fluorescence protein (GFP) gene amplified from pQBI63 (Quantum Biotechnologies), or maltose binding protein amplified from pMAL-c2 (New England Biolabs), resulting in plasmid pC³859-GST, pC³859-GFP, and pC³859-MBP respectively.

D. pC³876 and pC³878

The GST gene was PCR amplified with Taq DNA polymerase from plasmid pGEX-4T-2 with the following 3 primers:

#1-5′-GST(TetR): 5′aataaaaaactagtttgacaaataactctatcaatgatagagtgtcacaaaaaggagg  (SEQ ID NO:31)

#2-5′-GST(TetR): 5′gatagagtgtcaacaaaaaggaggaattaatgatgtcccctatactaggttattgg  (SEQ ID NO:32)

3′-GST(TetR): 5′ggattaaggtaaccttaatccgattttggaggatgg  (SEQ ID NO:33)

#2-5′-GST(TetR) can anneal to the 5′ end of GST and amplify the GST gene along with the 3′-GST(TetR) primer; #1-5′-GST(TetR) can anneal to the 5′ end of the resulting DNA fragment and extend the 5′ end sequence. The final PCR product contains the GST gene fused to the promoter region used to control TetR expression in pWH353 or pWH354, and it has an SpeI site at its 5′ end and a BstEII site at its 3′ end. The amplified DNA was purified with the Wizard PCR Preparation and Purification Kit (Promega) and cloned into the pT7Blue-3 vector (Novagen), resulting in a clone designated pT7Blue-3-GST. The cloned DNA fragment in pT7Blue-3-GST was then excised from the vector with restriction endonucleases SpeI and BstEII, gel purified, and cloned into the SpeI/BstEII sites of pWH353 and pWH354, resulting in plasmids pC³876 and pC³878, respectively. These two plasmids have the same sequence as pWH353 or pWH354, except that the GST open reading frame sequence has been substituted for the TetR open reading frame in the original plasmids.

E. pC³874, pC³875, and pC³875 Derivatives

As the multilinker cloning site in pT7Blue-3 is flanked by two EcoRI sites, the cloned GST DNA fragment in pT7Blue-3-GST was also excised from pT7Blue-3-GST with EcoRI digestion. The agarose gel-purified DNA fragment was then ligated to the EcoRI vector portion of pC³859 containing the origin of replication. The resulting plasmids pC³874 and pC³875 have the GST gene in alternative orientations.

The following DNA sequence, which encoded the JT01 peptide (DPNTWQLRWPMH; SEQ ID NO:34) followed by a Gly-Gly-Arg-Gly-Gly-Met (SEQ ID NO:35) linker, was inserted after the initiation ATG codon of the GST gene in pC³875, resulting in plasmid pC³882:

5′gatcctaatacatggcagttgaggtggcctatgcatggcggccgcggaggtatg  (SEQ ID NO:36).

The following DNA sequence, which encoded JT01 peptide flanked by a Ser-Ser dipeptide on each side (ssJT01ss) and followed by a Gly-Gly-Arg-Gly-Gly-Met linker, was inserted after the initiation ATG codon of the GST gene in pC³875, resulting in plasmid pC³883:

5′agctctgatcctaatacatggcagttgaggtggcctatgcattcttcaggcggccgcggaggtatg  (SEQ ID NO:37)

The following DNA sequence, which encoded a peptide with the sequence of Ser-Arg-Trp-Glu-Lys-Tyr-Ile-Asn-Ser-Phe-Glu-Leu-Asp-Ser-Arg-Gly-Gly-Arg-Gly-Gly-Met (LysN2), was inserted after the initiation ATG codon of the GST gene in pC³875, resulting in plasmid pC³888:

5′tctagatgggaaaaatatattaaffcttttgaattagattctcgaggtggtagaggtggaatg  (SEQ ID NO :38)

The following DNA sequence, which encoded a peptide with the sequence of Ser-Ser-Gln-Gly-Thr-Met-Arg-Trp-Phe-Asp-Tip-Tyr-Arg-Ser-Arg-Gly-Gly-Arg-Gly-Gly-Met (LysN3), was inserted after the initiation ATG codon of the GST gene in pC³875, resulting in plasmid pC³889:

 5′agctctcaaggtactatgagatggtttgattggtatagatctcgaggtggtagaggtggaatg  (SEQ ID NO:39)

F. pC³884 and pC³886

The green fluorescence protein (GFP) gene was amplified from plasmid pQBI63 (Quantum Biotechnologies Inc.) using the following oligonucleotides with Taq DNA polymerase:

5′GFP(NotI): 5′agcaccttggcggccgcggaggtgctagcaaaggagaagaactcttcac  (SEQ ID NO:40)

3′GFP(BstEII): 5′aactgaggtaacctcagttgtacagttcatccatgcc  (SEQ ID NO:41).

The amplified DNA was purified with the Wizard PCR Preparation and Purification Kit, and digested with restriction endonucleases NotI and BstEII. The digested DNA was gel purified and ligated to the NotI/BstEII sites of pC³882, resulting in pC³886.

The maltose binding protein (MBP) gene was amplified from plasmid pMAL-c2 (New England Biolabs) with Taq DNA polymerase using the following oligonucleotides:

5′MalE(NotI): 5′tttaccttggcggccgcggaggtaaactgaagaaggtaaactggtaatctgg  (SEQ ID NO:42);

3′MalE(BstEII): 5′acttagggtaaccttaagtctgc gcgtctttcagggcttc  (SEQ ID NO:43)

The amplified DNA was purified with the Wizard PCR Preparation and Purification Kit, and digested with restriction endonucleases NotI and BstEII. The digested DNA was gel purified and ligated to the NotI/BstEII sites of pC³882, resulting in pC³884.

Example 12

Characterization of Expression Plasmids

A. pWH353 and pWH354

pWH353 and pWH354 were transformed separately into S. aureus RN4220 cells by electroporation. The transformants were tested for inducible expression by growing in LB broth containing 30 μg/ml kanamycin. After the OD₆₀₀ reached 0.5, the cultures were split and tetracycline was added to one set of the cultures to a final concentration of 0.5 μg/ml. The induction was for 3 hours at 37° C. The S. aureus cells were then pelleted and resuspended in 80 mM Tris-HCl, pH 7.4 containing 200 μg/ml lysostaphin, incubated at 37° C. for 5 minutes, frozen and thawed twice in a dry ice/ethanol bath and in a 37° C. water bath. The samples were then sonicated twice and centrifuged at 14,000 g for 10 minutes, and the supernatants were collected and subjected to analysis on a 4-20% acrylamide gradient gel, by SDS-PAGE. The CAT activities in these samples were determined and summarized in Table 9. The results indicated that CAT is inducibly expressed, although the expression levels are not high enough to produce a visible band by Coomassie staining on an SDS polyacrylamide gel.

TABLE 9 Inducible expression of chloramphenicol acetyltransferase activity in S. aureus Plasmid Induction Relative CAT Activity No Plasmid − ND No Plasmid + ND pWH353 − 41 pWH353 + >4110 pWH354 − 1 pWH354 + 350 ND: Not detectable

Expression of GST with pWH353-2G in S. aureus RN4220 was also characterized. The transformants of pWH353-2G were tested for inducible expression by growing in LB broth containing 30 μg/ml kanamycin. After the OD₆₀₀ reached 0.5, the cultures were divided into 4 tubes and anhydrotetracycline was added to 0, 0.0625, 0.25, or 1 μg/ml. The induction was for 3 hours at 37° C. The S. aureus cells were then pelleted and resuspended in 1×PBS containing 100 μg/ml lysostaphin, incubated at 37° C. for 5 minutes, frozen and thawed twice on dry ice/ethanol and 37° C. water bath. The samples were then sonicated twice and centrifuged at 14,000 g for 10 minutes and the supernatants were collected and subjected to analysis on a 4-20% SDS-PAGE. The result indicated that GST expression level was very low, not detectable by Coomassie blue staining. Characterization of pWH354-2G, pWH353-2G-M1, and pWH354-2G-M1 transformants of RN4220 generated similar results.

B. Expression and Growth Characterization of S. aureus Cells Harboring pWH353(ΔtetR) and pWH354(ΔtetR), or pWH354-2G(ΔtetR)

pWH353(ΔtetR), pWH354(ΔtetR) and pWH354-2G(ΔtetR) were transformed into separate cultures of S. aureus CYL316t cells by electroporation. Overnight cultures of CYL316t/pWH353(ΔtetR), CYL316t/pWH354(ΔtetR) and CYL316t/pWH354-2G(ΔtetR) were diluted 100-fold in fresh LB with 5 μg/ml erythromycin and 25 μg/ml kanamycin. After the OD₆₀₀ reached 0.5, the cultures were split into two tubes, and anhydrotetracycline was added to one set of the cultures to a final concentration of 0.2 μg/ml. The induction was for 3 hours at 37° C. The S. aureus cells were then pelleted and resuspended in 1×PBS containing 100 μg/ml lysostaphin, incubated at 37° C. for 5 minutes, frozen and thawed twice in a dry ice/ethanol bath and in a 37° C. water bath. The samples were then sonicated twice and centrifuged at 14,000 g for 10 minutes. The supernatants were collected and subjected to analysis on a 4-20% acrylamide gradient gel, by SDS-PAGE. Cultures of CYL316t/pWH353(ΔtetR) were also tested, with similar results. The results indicated that removing the tetR gene from the pMM353 and pWH354 plasmids drastically increased efficiency of CAT expression, to the point where a protein band became visible by Coomassie blue staining on an SDS-polyacrylamide gel, while GST expression was not significantly enhanced.

Growth of CYL316t, CYL316t/pWH353(ΔtetR), CYL316t/pWH354(ΔtetR) and CYL316t/pWH354-2G(ΔtetR) was also characterized by the following experiment. Overnight cultures of CYL316t, CYL316t/pWH353(ΔtetR), CYL316t/pWH354(ΔtetR) and CYL316t/pWH354-2G(ΔtetR) were diluted 100-fold in fresh tryptone-soy broth (TSB; Difco) containing 5 μg/ml erythromycin and 25 μg/ml kanamycin (for CYL316t/pWH353(ΔtetR), CYL316t/pWH354(ΔtetR) and CYL316t/pWH354-2G(ΔtetR)). After growing for 1 hour at 37° C., each culture was split into two tubes and to one anhydrotetracycline was added to 200 ng/ml. The OD₆₀₀'s of the cultures were then recorded at various time points. The results depicted in FIGS. 11A-11D demonstrate that anhydrotetracycline induction caused a significant inhibition in S. aureus growth.

C. Growth Characterization of S. aureus Cells Carrying pC³859

An overnight culture of CYL316t/pC³859 was diluted 100-fold in fresh TSB with 5 μg/ml erythromycin and 25 μg/ml kanamycin. After growing for 1 hour at 37° C., the culture was split into two tubes. To one, anhydrotetracycline was added to 200 ng/ml. The OD₆₀₀ of the culture was then recorded at various time points. The result is depicted in FIG. 11E, which demonstrates that the anhydrotetracycline induction related cytotoxicity observed with S. aureus cells carrying pWH353(ΔtetR) or pWH354(ΔtetR) is much reduced for S. aureus cells harboring pC³859, by comparison.

D. Expression Characterization of E. coli or S. aureus Cells Harboring pC³859 or its Derivatives

Plasmids pC³859, pC³859-GST, pC³859-GFP, and pC³859-MBP were each transformed into separate cultures of E. coli DH5αPRO and S. aureus CYL316t. Overnight cultures of the resulting E. coli or S. aureus transformant cells were diluted 100-fold in fresh LB with 50 μg/ml spectinomycin and 25 μg/ml kanamycin for E. coli cells or LB with 5 μg/ml erythromycin and 25 μg/ml kanamycin for S. aureus cells. After the OD₆₀₀ reached 0.5, the cultures were split into two tubes, and anhydrotetracycline was added to one set of the cultures to 0.2 μg/ml. The induction was for 3 hours at 37° C. The E. coli cells were pelleted and boiled 5 minutes in SDS-PAGE sample buffer. The S. aureus cells were pelleted and resuspended in 1×PBS containing 100 μg/ml lysostaphin, incubated at 37° C. for 5 minutes, frozen and thawed twice in a dry ice/ethanol bath and in a 37° C. water bath. The samples were then sonicated twice and centrifuged at 14,000 g for 10 minutes and the supernatants were mixed with SDS-PAGE sample buffer and boiled for 5 minutes. The samples were subjected to analysis on a 4-20% acrylamide gradient gel, by SDS-PAGE. The results indicated that while CAT can be efficiently expressed from pC³859 in S. aureus, other genes, such as GST, GFP, or MBP are not efficiently expressed using the same genetic background.

E. Expression Characterization of S. aureus Cells Harboring pC³876, pC³878, pC³874, or pC³875

Plasmids pC³876, pC³878, pC³874, and pC³875 were transformed into separate cultures of S. aureus CYL316t. Overnight cultures of the resulting S. aureus cells were diluted 100-fold in fresh LB with 5 μg/ml erythromycin and 25 μg/ml kanamycin for S. aureus cells. After the OD₆₀₀ reached 0.5, the cultures were split into two tubes and anhydrotetracycline was added to one set of the cultures to 0.2 μg/ml. The induction was for 3 hours at 37° C. The S. aureus cells were pelleted and resuspended in 80 mM Tris-HCl, pH 7.4 containing 200 μg/ml lysostaphin, incubated at 37° C. for 5 minutes, frozen and thawed twice in a dry ice/ethanol bath and in a 37° C. water bath. The samples were then sonicated twice and centrifuged at 14,000 g for 10 minutes and the supernatants were mixed with SDS-PAGE sample buffer and boiled for 5 minutes. The samples were subjected to analysis on a 4-20% acrylamide gradient gel, by SDS-PAGE (FIG. 12). The results indicated that GST gene can be efficiently expressed with these constructs. GST from the above cell lysates was also purified with glutathione-Sepharose, and analyzed by SDS-PAGE. The results indicated that pC³875 had the lowest level of basal expression from the tet promoter/operator.

F. Expression Characterization of pC³882 and pC³883 in CYL316t

Plasmid pC³882 and pC³883 were transformed into separate cultures of S. aureus CYL316t. Overnight cultures of the resulting S. aureus transformant cells were diluted 100-fold in fresh LB with 5 μg/ml erythromycin and 25 μg/ml kanamycin. After the OD₆₀₀ reached 0.5, the cultures were split into two tubes and anhydrotetracycline was added to one set of the cultures to 0.2 μg/ml. The induction was for 3 hours at 37° C. The S. aureus cells were pelleted and resuspended in 1×PBS containing 100 μg/ml lysostaphin, incubated at 37° C. for 5 minutes, and frozen and thawed twice in a dry ice/ethanol bath and in a 37° C. water bath. The samples were then sonicated twice and centrifuged at 14,000 g for 10 minutes, and the supernatants were mixed with SDS-PAGE sample buffer and boiled for 5 minutes. The samples were subjected to analysis on a 4-20% acrylamide gradient gel, by SDS-PAGE. The results indicated that both JT01-GST and ssJT01ss could be efficiently expressed, and the expression for JT01-GST was significantly higher than ssJT01ss-GST. With an ELISA assay (Pharmacia; product #27-4592), it was determined that expression of JT01-GST increased about 1000-fold upon anhydrotetracycline induction.

JT01-GST and ssJT01ss-GST fusion polypeptides were also purified from the above described cell extracts using glutathione affinity resin. The purified peptide-GST fusion polypeptides were tested for their inhibitory activity on S. aureus MetRS with 2 mM ATP, 50 μM methionine, and 90 μM E. coli total tRNA as the substrates. Under these conditions, JT101-GST only inhibited 60% of S. aureus activity with an IC₅₀ (peptide concentration required to inhibit 50% MetRS activity) about 15 nM, while both free JT101 peptide and ssJT01ss-GST could fully inhibitS. aureus MetRS activity with an IC₅₀ of about 100 nM.

G. Growth Characterization of S. aureus Cells Carrying Plasmid pC³882 or pC³883

Plasmids pC³882 and pC³883 were transformed separately into S. aureus strains CYL316t and CYL316tt. Overnight cultures of the resulting S. aureus transformant cells grown in TSB with 5 μg/ml erythromycin and 25 μg/ml kanamycin were diluted 100-fold in fresh TSB with 5 μg/ml erythromycin and 25 μg/ml kanamycin. After growing for 1 hour at 37° C., the culture was split into two tubes and to one, anhydrotetracycline (for CYL316t strains) or tetracycline (for CYL316tt strains) was added to 200 μg/ml and 1 μg/ml, respectively. The OD₆₀₀ of the culture was then recorded at various time points. The results depicted in FIGS. 13A, 13B, and 13C demonstrate that induction of expression of JT01-GST or ssJT01ss-GST, but not GST alone, caused significant inhibition of S. aureus growth. Between the two fusion polypeptides, ssJT01ss-GST expression had the stronger growth inhibitory effect.

H. Expression of JT01-GFP, JT01-MBP Fusions

Plasmid pC³884 and pC³886 were each transformed separately into separate cultures of S. aureus CYL316tt. Overnight cultures of the resulting transformant S. aureus cells were diluted 100-fold in fresh LB with 5 μg/ml erythromycin and 25 μg/ml kanamycin. After the OD₆₀₀ reached 0.5, the cultures were split into two tubes and tetracycline was added to one set of the cultures to 1 μg/ml. The induction was for 3 hours at 37° C. The S. aureus cells were pelleted and resuspended in 1×PBS containing 100 μg/ml lysostaphin, incubated at 37° C. for 5 minutes, frozen and thawed twice in a dry ice/ethanol bath and in a 37° C. water bath. The samples were then sonicated twice and centrifuged at 14,000 g for 10 minutes and the supernatants were mixed with SDS-PAGE sample buffer and boiled for 5 minutes. Analysis of the resulting samples on a SDS-polyacrylamide gel revealed that both JT01-GFP and JT01-MBP polypeptides were efficiently expressed.

I. Expression of LysN2-GST, and LysN3-GST Fusions

Plasmid pC³888 or pC³889 was transformed into S. aureus CYL316t. Overnight cultures of the resulting S. aureus cells were diluted 100-fold in fresh LB with 5 μg/ml erythromycin and 25 μg/ml kanamycin. After the OD₆₀₀ reached 0.5, the cultures were split into two tubes and anhydrotetracycline was added to one set of the cultures to 0.2 μg/ml. The induction was for 3 hours at 37° C. The S. aureus cells were pelleted and resuspended in 1×PBS containing 100 μg/ml lysostaphin, incubated at 37° C. for 5 minutes, frozen and thawed twice in a dry ice/ethanol bath and in a 37° C. water bath. The samples were then sonicated twice and centrifuged at 14,000 g for 10 minutes, and the supernatants were mixed with SDS-PAGE sample buffer and boiled for 5 minutes. Examination of the resulting samples on a SDS-polyacrylamide gel revealed that both LysN2-GST and LysN3-GST were efficiently expressed.

Example 13

Protection of a Lethal S. aureus Infection by Intracellular Expression of a Peptide In Vivo

In experiments using S. aureus cells grown in culture, intracellularly expressed ssJT01ss bound to and inhibited a specific essential cellular target in a manner similar to that of an antimicrobial drug. Hence induction of expression of this peptide during an infection should have the effect of an antibiotic. An established animal infection model was used to test this concept (Onyegji, C. O. et al., Antimicrobial Agents and Chemotherapy 38:112-117, 1994).

Six groups of CD-1 female mice (5 mice per group, Charles River Laboratories, Wilmington, Mass.) weighing 20-24 grams were used in this experiment. The inoculum was prepared from CYL316tt/pC³883 (encoding a ssJT01ss peptide-GST fusion protein under the control of the tet operon) which was cultured at 37° C. for 17 hours in TS broth containing erythromycin and kanamycin. 1.6×10¹⁰ cfu (colony-forming units) of S. aureus CYL316tt/pC³883 (OD₆₀₀ of 0.1=10⁸ cfu/ml) from the overnight culture were diluted to 20 ml with 0.01 M PBS (Sigma P-0261) containing 8% hog gastric mucin (Sigma M-2378) as well as 50 μg/ml kanamycin and 10 μg/ml erythromycin. Each mouse of groups 1 through 4 was injected with 0.5 ml of the inoculum intraperitoneally (i.p.), equivalent to 4×10⁸ cfu/mouse (lethal dose). Groups 5 and 6 served as vector controls. Each mouse of these two groups was injected with 4×10⁸ cfu of CYL316tt/pC³875, which was cultured and processed the same way as CYL316tt/pC³883. One half hour and four hours after the inoculation, groups 1 and 5 received a saline injection i.p. at 10 ml/kg; groups 2 and 6 received i.p. injections of tetracycline (Sigma T-3383) at 8 mg/kg; group 3 received i.p. injections of tetracycline at 4 mg/kg; group 4 received i.p. injections of ciprofloxacin (Bayer 851510, dissolved in water) at 50 mg/kg. The injection volume for all the animals was 10 ml/kg. Surviving mice were counted at 7 days post inoculation. Ciprofloxacin given at 50 mg/kg protected all infected animals from lethal infection.

The data summarized in Table 10 demonstrate that induction of intracellular expression of the ssJT01ss peptide can be achieved in an animal infection. Inhibition of S. aureus MetRS by the intracellularly expressed ssJT01ss peptide cured a lethal infection in a mouse model.

TABLE 10 Inhibition of S. aureus growth in mice by intracellular production of S. aureus MetRS inhibitor # of Mice # of Mice Experimental Condition Tested Survival M1 Peptide Saline, 10 mg/kg C2 5 0 Inducer, mg/kg C2 5 5 Expression Control Saline, 10 mg/kg C2 5 0 Inducer, mg/kg C2 5 0

The relevant teachings of all references cited herein are hereby incorporated by reference herein in their entirety.

While this invention has been particularly shown and described with references to preferred embodiments thereof, it will be understood by those skilled in the art that various changes in form and details may be made therein without departing from the spirit and scope of the invention as defined by the appended claims.

46 1 15 PRT Artificial Sequence Synthetic Peptide 1 Ser Arg Asp Trp Gly Phe Trp Asp Trp Gly Val Asp Arg Ser Arg 1 5 10 15 2 16 PRT Artificial Sequence Synthetic Peptide 2 Ser Arg Asp Trp Gly Phe Trp Arg Leu Pro Glu Ser Met Ala Ser Arg 1 5 10 15 3 15 PRT Artificial Sequence Synthetic Peptide 3 Ser Arg Glu Trp His Phe Trp Arg Asp Tyr Asn Pro Thr Ser Arg 1 5 10 15 4 15 PRT Artificial Sequence Synthetic Peptide 4 Ser Ser Glu Arg Gly Ser Gly Asp Arg Gly Glu Lys Gly Ser Arg 1 5 10 15 5 43 DNA Synthetic PCR Primer 5 ccaacaacat atgtcccgtg aatggcactt ctggcgtgac tac 43 6 57 DNA Synthetic PCR Primer 6 ttctggcgtg actacaaccc gacctcccgt gggggtggag gcatgtcccc tatacta 57 7 32 DNA Synthetic PCR Primer 7 agttgaattc ttaatccgat tttggaggat gg 32 8 28 DNA Synthetic PCR Primer 8 caaggtaccc atgtcccgtg aatggcac 28 9 31 DNA Synthetic PCR Primer 9 cgcggatcct taatccgatt ttggaggatg g 31 10 31 DNA Synthetic PCR Primer 10 aatccgctcg aggattattg ctattggtgc c 31 11 33 DNA Synthetic PCR Primer 11 aatcgtaagc ttttatttta agttatcata ttt 33 12 22 PRT Artificial Sequence Synthetic Peptide 12 Ser Arg Glu Trp His Phe Trp Arg Asp Tyr Asn Pro Thr Ser Arg Gly 1 5 10 15 Gly Lys Phe Ile Thr Cys 20 13 12 PRT Artificial Sequence Synthetic Peptide 13 Asp Pro Asn Thr Trp Gln Leu Arg Trp Pro Met His 1 5 10 14 12 PRT Artificial Sequence Synthetic Peptide 14 Met Trp Asp Leu Pro Tyr Ile Trp Ser Arg Pro Val 1 5 10 15 12 PRT Artificial Sequence Synthetic Peptide 15 Ala Asp Thr Leu Asn Trp Tyr Tyr Tyr Ala Ser Trp 1 5 10 16 12 PRT Artificial Sequence Synthetic Peptide 16 Ala Asn Asn Leu Ser Thr Met Lys Lys Leu Lys Gln 1 5 10 17 19 PRT Artificial Sequence Synthetic Peptide 17 Asp Pro Asn Thr Trp Gln Leu Arg Trp Pro Met His Gly Gly Lys Phe 1 5 10 15 Ile Thr Cys 18 33 DNA Synthetic PCR Primer 18 acgggtcgac tcatatcttt tattcaataa tcg 33 19 32 DNA Synthetic PCR Primer 19 ccggaaagct tacttattaa ataatttata gc 32 20 34 DNA Synthetic PCR Primer 20 taagtaagct taaggaggaa ttaatgatgt ctag 34 21 34 DNA Synthetic PCR Primer 21 acgggtcgac ttaagaccca ctttcacatt taag 34 22 45 DNA Synthetic PCR Primer 22 ctcggtaccg agctaaaatt cggaggcata tcaaatgagc tctgg 45 23 44 DNA Synthetic PCR Primer 23 ggcatatcaa atgagctctg gaggtggagg catgtcccct atac 44 24 31 DNA Synthetic PCR Primer 24 aggcctaggt taatccgatt ttggaggatg g 31 25 42 DNA Synthetic Cloning Fragment 25 ctgatccgaa tacgtggcag ttgcggtggc ctatgcatag ct 42 26 42 DNA Synthetic Cloning Fragment 26 atgcataggc caccgcaact gccacgtatt cggatcagag ct 42 27 48 DNA Synthetic Cloning Fragment 27 tcgagttcat gaaaaactaa aaaaaatatt gacatcccta tcagtgat 48 28 45 DNA Synthetic Cloning Fragment 28 agagataatt aaaataatcc ctatcagtga tagagagctt gcatg 45 29 29 DNA Synthetic Cloning Fragment 29 caagctctct atcactgata gggattatt 29 30 56 DNA Synthetic Cloning Fragment 30 ttaattatct ctatcactga tagggatgtc aatatttttt ttagtttttc atgaac 56 31 58 DNA Synthetic PCR Primer 31 aataaaaaac tagtttgaca aataactcta tcaatgatag agtgtcacaa aaaggagg 58 32 56 DNA Synthetic PCR Primer 32 gatagagtgt caacaaaaag gaggaattaa tgatgtcccc tatactaggt tattgg 56 33 36 DNA Synthetic PCR Primer 33 ggattaaggt aaccttaatc cgattttgga ggatgg 36 34 12 PRT Artificial Sequence Synthetic Peptide 34 Asp Pro Asn Thr Trp Gln Leu Arg Trp Pro Met His 1 5 10 35 6 PRT Artificial Sequence Synthetic Peptide 35 Gly Gly Arg Gly Gly Met 1 5 36 54 DNA Artificial Sequence Synthetic Cloning Fragment 36 gatcctaata catggcagtt gaggtggcct atgcatggcg gccgcggagg tatg 54 37 66 DNA Artificial Sequence Synthetic Cloning Fragment 37 agctctgatc ctaatacatg gcagttgagg tggcctatgc attcttcagg cggccgcgga 60 ggtatg 66 38 22 PRT Artificial Sequence Synthetic Peptide 38 Ser Arg Trp Glu Lys Tyr Ile Asn Ser Phe Glu Leu Asp Ser Arg Gly 1 5 10 15 Gly Arg Gly Gly Met Lys 20 39 63 DNA Artificial Sequence Synthetic Cloning Fragment 39 tctagatggg aaaaatatat taattctttt gaattagatt ctcgaggtgg tagaggtgga 60 atg 63 40 22 PRT Artificial Sequence Synthetic Peptide 40 Ser Ser Gln Gly Thr Met Arg Trp Phe Asp Trp Tyr Arg Ser Arg Gly 1 5 10 15 Gly Arg Gly Gly Met Lys 20 41 63 DNA Artificial Sequence Synthetic Cloning Fragment 41 agctctcaag gtactatgag atggtttgat tggtatagat ctcgaggtgg tagaggtgga 60 atg 63 42 49 DNA Artificial Sequence Synthetic PCR Primer 42 agcaccttgg cggccgcgga ggtgctagca aaggagaaga actcttcac 49 43 37 DNA Artificial Sequence Synthetic PCR Primer 43 aactgaggta acctcagttg tacagttcat ccatgcc 37 44 52 DNA Artificial Sequence Synthetic Cloning Fragment 44 tttaccttgg cggccgcgga ggtaaactga agaaggtaaa ctggtaatct gg 52 45 40 DNA Artificial Sequence Synthetic Cloning Fragment 45 acttagggta accttaagtc tgcgcgtctt tcagggcttc 40 46 4 PRT Artificial Sequence linker peptide 46 Glu Gly Gly Gly 1 

What is claimed is:
 1. A method for identifying one or more compounds that are candidates for binding to a target cell component in a pathogen and inhibiting infection of a mammal by the pathogen comprising: a) constructing a pathogen comprising a regulable gene encoding a biomolecule which binds to the target cell component; b) infecting one or more test animals with the constructed pathogen, and one or more control animals with the constructed pathogen or with a control pathogen; c) regulating expression of the regulable gene to produce the biomolecule in the test animals; d) monitoring the test animals and the control animals for signs of infection, wherein observing fewer or less severe signs of infection in the test animals than in the control animals indicates that the biomolecule is a biomolecular inhibitor of infection by the pathogen; and e) identifying one or more compounds that compete with the biomolecular inhibitor of infection for binding to the target cell component in a competitive binding assay; whereby, if a compound competes with the biomolecular inhibitor of infection for binding to the target cell component, then the compound is a candidate for binding to the target cell component in the pathogen and inhibiting infection of a mammal by the pathogen.
 2. The method of claim 1 wherein the pathogen is Staphylococcus aureus.
 3. The method of claim 1 wherein the pathogen is Staphylococcus aureus and the target cell component is Staphylococcus aureus methionyl-tRNA synthetase.
 4. A method for identifying one or more compounds that are candidates for binding to a target cell component in a pathogen and inhibiting infection of a mammal by the pathogen, comprising: a) constructing a pathogen comprising a regulable gene encoding a biomolecule which binds to the target cell component; b) regulating expression of the gene in a culture of constructed pathogen cells, thereby producing the biomolecule in the constructed pathogen cells; c) monitoring growth of the constructed pathogen cells in culture, relative to growth of suitable control cells, whereby, if growth is decreased in the constructed pathogen cells, compared to growth of the control cells, then the biomolecule is a biomolecular inhibitor of growth; d) infecting one or more test animals with the constructed pathogen, and one or more control animals with the constructed pathogen or with a control pathogen; e) regulating expression of the regulable gene, thereby producing the biomolecule in the constructed pathogen in the test animals; f) monitoring the test animals and the control animals for signs of infection, wherein observing fewer or less severe signs of infection in the test animals than in the control animals indicates that the biomolecule is a biomolecular inhibitor of infection by the pathogen; and g) identifying one or more compounds that compete with the biomolecular inhibitor of infection for binding to the target cell component; whereby, if a compound competes with the biomolecular inhibitor of infection for binding to the target cell component, then the compound is a candidate for binding to the target cell component in the pathogen and inhibiting infection of the mammal by the pathogen.
 5. The method of claim 4 wherein the pathogen is Staphylococcus aureus.
 6. The method of claim 4 wherein the pathogen is Staphylococcus aureus and the target cell component is Staphylococcus aureus methionyl-tRNA synthetase.
 7. A method for identifying a biomolecular inhibitor of infection by a pathogen, comprising: a) in pathogen cells comprising a biomolecule and a cell component, wherein the biomolecule is a biomolecular binder of the cell component, and expression of the gene encoding the biomolecule is regulable, regulating expression of the gene, thereby producing the biomolecule; b) monitoring growth of the pathogen cells in culture relative to growth of control cells, whereby, if growth is decreased in the pathogen cells compared to growth of the control cells, then the biomolecule is a biomolecular inhibitor of growth of the pathogen cells; c) infecting one or more test animals with the pathogen cells in which growth was decreased compared to the control cells in step b); d) infecting one or more control animals with the pathogen cells of step c) or with a control pathogen; e) regulating expression of the gene in the test animals, thereby producing the biomolecule in the test animals; and f) monitoring said test animals and said control animals for signs of infection; wherein observing fewer or less severe signs of infection in said test animals compared to signs of infection in the control animals indicates that the biomolecule is a biomolecular inhibitor of infection by the pathogen.
 8. The method of claim 7 wherein the pathogen cells are Staphylococcus aureus cells.
 9. The method of claim 7 wherein the pathogen cells are Staphylococcus aureus cells and the cell component is Staphylococcus aureus methionyl-tRNA synthetase. 